net: usb: smsc75xx: Fix uninit-value access in __smsc75xx_read_reg

In the Linux kernel, the following vulnerability has been resolved:

net: usb: smsc75xx: Fix uninit-value access in __smsc75xx_read_reg

syzbot reported the following uninit-value access issue:

=====================================================
BUG: KMSAN: uninit-value in smsc75xx_wait_ready drivers/net/usb/smsc75xx.c:975 [inline]
BUG: KMSAN: uninit-value in smsc75xx_bind+0x5c9/0x11e0 drivers/net/usb/smsc75xx.c:1482
CPU: 0 PID: 8696 Comm: kworker/0:3 Not tainted 5.8.0-rc5-syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
Workqueue: usb_hub_wq hub_event
Call Trace:
__dump_stack lib/dump_stack.c:77 [inline]
dump_stack+0x21c/0x280 lib/dump_stack.c:118
kmsan_report+0xf7/0x1e0 mm/kmsan/kmsan_report.c:121
__msan_warning+0x58/0xa0 mm/kmsan/kmsan_instr.c:215
smsc75xx_wait_ready drivers/net/usb/smsc75xx.c:975 [inline]
smsc75xx_bind+0x5c9/0x11e0 drivers/net/usb/smsc75xx.c:1482
usbnet_probe+0x1152/0x3f90 drivers/net/usb/usbnet.c:1737
usb_probe_interface+0xece/0x1550 drivers/usb/core/driver.c:374
really_probe+0xf20/0x20b0 drivers/base/dd.c:529
driver_probe_device+0x293/0x390 drivers/base/dd.c:701
__device_attach_driver+0x63f/0x830 drivers/base/dd.c:807
bus_for_each_drv+0x2ca/0x3f0 drivers/base/bus.c:431
__device_attach+0x4e2/0x7f0 drivers/base/dd.c:873
device_initial_probe+0x4a/0x60 drivers/base/dd.c:920
bus_probe_device+0x177/0x3d0 drivers/base/bus.c:491
device_add+0x3b0e/0x40d0 drivers/base/core.c:2680
usb_set_configuration+0x380f/0x3f10 drivers/usb/core/message.c:2032
usb_generic_driver_probe+0x138/0x300 drivers/usb/core/generic.c:241
usb_probe_device+0x311/0x490 drivers/usb/core/driver.c:272
really_probe+0xf20/0x20b0 drivers/base/dd.c:529
driver_probe_device+0x293/0x390 drivers/base/dd.c:701
__device_attach_driver+0x63f/0x830 drivers/base/dd.c:807
bus_for_each_drv+0x2ca/0x3f0 drivers/base/bus.c:431
__device_attach+0x4e2/0x7f0 drivers/base/dd.c:873
device_initial_probe+0x4a/0x60 drivers/base/dd.c:920
bus_probe_device+0x177/0x3d0 drivers/base/bus.c:491
device_add+0x3b0e/0x40d0 drivers/base/core.c:2680
usb_new_device+0x1bd4/0x2a30 drivers/usb/core/hub.c:2554
hub_port_connect drivers/usb/core/hub.c:5208 [inline]
hub_port_connect_change drivers/usb/core/hub.c:5348 [inline]
port_event drivers/usb/core/hub.c:5494 [inline]
hub_event+0x5e7b/0x8a70 drivers/usb/core/hub.c:5576
process_one_work+0x1688/0x2140 kernel/workqueue.c:2269
worker_thread+0x10bc/0x2730 kernel/workqueue.c:2415
kthread+0x551/0x590 kernel/kthread.c:292
ret_from_fork+0x1f/0x30 arch/x86/entry/entry_64.S:293

Local variable ----buf.i87@smsc75xx_bind created at:
__smsc75xx_read_reg drivers/net/usb/smsc75xx.c:83 [inline]
smsc75xx_wait_ready drivers/net/usb/smsc75xx.c:968 [inline]
smsc75xx_bind+0x485/0x11e0 drivers/net/usb/smsc75xx.c:1482
__smsc75xx_read_reg drivers/net/usb/smsc75xx.c:83 [inline]
smsc75xx_wait_ready drivers/net/usb/smsc75xx.c:968 [inline]
smsc75xx_bind+0x485/0x11e0 drivers/net/usb/smsc75xx.c:1482

This issue is caused because usbnet_read_cmd() reads less bytes than requested
(zero byte in the reproducer). In this case, 'buf' is not properly filled.

This patch fixes the issue by returning -ENODATA if usbnet_read_cmd() reads
less bytes than requested.