serial: mxs-auart: add spinlock around changing cts state

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

serial: mxs-auart: add spinlock around changing cts state

The uart_handle_cts_change() function in serial_core expects the caller

to hold uport->lock. For example, I have seen the below kernel splat,

when the Bluetooth driver is loaded on an i.MX28 board.

[ 85.119255] ------------[ cut here ]------------

[ 85.124413] WARNING: CPU: 0 PID: 27 at /drivers/tty/serial/serial_core.c:3453 uart_handle_cts_change+0xb4/0xec

[ 85.134694] Modules linked in: hci_uart bluetooth ecdh_generic ecc wlcore_sdio configfs

[ 85.143314] CPU: 0 PID: 27 Comm: kworker/u3:0 Not tainted 6.6.3-00021-gd62a2f068f92 #1

[ 85.151396] Hardware name: Freescale MXS (Device Tree)

[ 85.156679] Workqueue: hci0 hci_power_on [bluetooth]

(...)

[ 85.191765] uart_handle_cts_change from mxs_auart_irq_handle+0x380/0x3f4

[ 85.198787] mxs_auart_irq_handle from __handle_irq_event_percpu+0x88/0x210

(...)