wifi: mt76: mt7921e: fix rmmod crash in driver reload test

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

wifi: mt76: mt7921e: fix rmmod crash in driver reload test

In insmod/rmmod stress test, the following crash dump shows up immediately.
The problem is caused by missing mt76_dev in mt7921_pci_remove(). We
should make sure the drvdata is ready before probe() finished.

[168.862789] ==================================================================
[168.862797] BUG: KASAN: user-memory-access in try_to_grab_pending+0x59/0x480
[168.862805] Write of size 8 at addr 0000000000006df0 by task rmmod/5361
[168.862812] CPU: 7 PID: 5361 Comm: rmmod Tainted: G OE 5.19.0-rc6 #1
[168.862816] Hardware name: Intel(R) Client Systems NUC8i7BEH/NUC8BEB, 05/04/2020
[168.862820] Call Trace:
[168.862822] <TASK>
[168.862825] dump_stack_lvl+0x49/0x63
[168.862832] print_report.cold+0x493/0x6b7
[168.862845] kasan_report+0xa7/0x120
[168.862857] kasan_check_range+0x163/0x200
[168.862861] __kasan_check_write+0x14/0x20
[168.862866] try_to_grab_pending+0x59/0x480
[168.862870] __cancel_work_timer+0xbb/0x340
[168.862898] cancel_work_sync+0x10/0x20
[168.862902] mt7921_pci_remove+0x61/0x1c0 [mt7921e]
[168.862909] pci_device_remove+0xa3/0x1d0
[168.862914] device_remove+0xc4/0x170
[168.862920] device_release_driver_internal+0x163/0x300
[168.862925] driver_detach+0xc7/0x1a0
[168.862930] bus_remove_driver+0xeb/0x2d0
[168.862935] driver_unregister+0x71/0xb0
[168.862939] pci_unregister_driver+0x30/0x230
[168.862944] mt7921_pci_driver_exit+0x10/0x1b [mt7921e]
[168.862949] __x64_sys_delete_module+0x2f9/0x4b0
[168.862968] do_syscall_64+0x38/0x90
[168.862973] entry_SYSCALL_64_after_hwframe+0x63/0xcd

Test steps:
1. insmode
2. do not ifup
3. rmmod quickly (within 1 second)