Bluetooth: btnxpuart: Fix btnxpuart_close

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

Bluetooth: btnxpuart: Fix btnxpuart_close

Fix scheduling while atomic BUG in btnxpuart_close(), properly

purge the transmit queue and free the receive skb.

[ 10.973809] BUG: scheduling while atomic: kworker/u9:0/80/0x00000002

...

[ 10.980740] CPU: 3 PID: 80 Comm: kworker/u9:0 Not tainted 6.8.0-rc7-0.0.0-devel-00005-g61fdfceacf09 #1

[ 10.980751] Hardware name: Toradex Verdin AM62 WB on Dahlia Board (DT)

[ 10.980760] Workqueue: hci0 hci_power_off [bluetooth]

[ 10.981169] Call trace:

...

[ 10.981363] uart_update_mctrl+0x58/0x78

[ 10.981373] uart_dtr_rts+0x104/0x114

[ 10.981381] tty_port_shutdown+0xd4/0xdc

[ 10.981396] tty_port_close+0x40/0xbc

[ 10.981407] uart_close+0x34/0x9c

[ 10.981414] ttyport_close+0x50/0x94

[ 10.981430] serdev_device_close+0x40/0x50

[ 10.981442] btnxpuart_close+0x24/0x98 [btnxpuart]

[ 10.981469] hci_dev_close_sync+0x2d8/0x718 [bluetooth]

[ 10.981728] hci_dev_do_close+0x2c/0x70 [bluetooth]

[ 10.981862] hci_power_off+0x20/0x64 [bluetooth]