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]

Fixes: 689ca16e

 ("Bluetooth: NXP: Add protocol support for NXP Bluetooth chipsets")
Cc: stable@vger.kernel.org
Signed-off-by: Marcel Ziswiler <marcel.ziswiler@toradex.com>
Reviewed-by: Neeraj Sanjay Kale <neeraj.sanjaykale@nxp.com>
Signed-off-by: Francesco Dolcini <francesco.dolcini@toradex.com>
Signed-off-by: Luiz Augusto von Dentz <luiz.von.dentz@intel.com>