Bluetooth: hci_qca: Do not write to the serial port after it is closed

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

Bluetooth: hci_qca: Do not write to the serial port after it is closed

hci_uart_close() closes the serdev port if HCI_QUIRK_NON_PERSISTENT_SETUP
is set (for example, for the WCN399x family). A failed hci_dev_open_sync()
following a successful qca_setup() calls hdev->close() but not
hdev->shutdown(), so the port is closed while power->vregs_on is left true.
qca_serdev_remove() then passes its power->vregs_on test and calls
qca_power_off(), which writes to the closed port unconditionally.

Seen on a WCN3988 by unbinding the driver after a controller failure. The
trace below is from a 7.0.0 based kernel, where qca_power_off() was still
named qca_power_shutdown():

Unable to handle kernel NULL pointer dereference at virtual address
0000000000000038
Call trace:
tty_set_termios+0x50/0x238 (P)
ttyport_set_baudrate+0x84/0xc0
serdev_device_set_baudrate+0x24/0x40
qca_power_shutdown+0x158/0x1fc [hci_uart]
qca_serdev_remove+0x54/0x68 [hci_uart]
serdev_drv_remove+0x1c/0x2c
device_remove+0x4c/0x80
device_release_driver_internal+0x1cc/0x224
device_driver_detach+0x18/0x24
unbind_store+0xb4/0xc0

Check HCI_UART_PROTO_READY, which hci_uart_close() clears in the same place
it closes the port, before writing to it. The regulator disable is left
unconditional so the controller is still powered down.

The dangling serport->tty that turns this into a use-after-free is
addressed in a separate patch.