serial: max3100: Lock port->lock when calling uart_handle_cts_change()

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

serial: max3100: Lock port->lock when calling uart_handle_cts_change()

uart_handle_cts_change() has to be called with port lock taken,

Since we run it in a separate work, the lock may not be taken at

the time of running. Make sure that it's taken by explicitly doing

that. Without it we got a splat:

WARNING: CPU: 0 PID: 10 at drivers/tty/serial/serial_core.c:3491 uart_handle_cts_change+0xa6/0xb0

...

Workqueue: max3100-0 max3100_work [max3100]

RIP: 0010:uart_handle_cts_change+0xa6/0xb0

...

max3100_handlerx+0xc5/0x110 [max3100]

max3100_work+0x12a/0x340 [max3100]