ENGR00178951-1 serial/imx: dead lock dedected when handle sysrq

ENGR00178951-1 serial/imx: dead lock dedected when handle sysrq

imx_rxint -> spin_lock_irqsave(&sport->port.lock, flags);

And then it will call the following functions:

uart_handle_sysrq_char -> handle_sysrq -> printk ->
__call_console_write_drivers -> imx_console_write ->

Here the imx_console_write function will call:
spin_lock_irqsave(&sport->port.lock, flags);

The A-A deadlock happens. We need spin_unlock before handle sysrq char
and spin_lock again after it.

Signed-off-by: Jason Liu <r64343@freescale.com>