imx: serial: be sure to stop xmit upon shutdown

needed to avoid continued transmission by hardware
while software already shuts down, which might
cause dangling characters to show up in hardware
queues when restarting the device.

Signed-off-by: Fabian Godehardt <fg@emlix.com>
Signed-off-by: Oskar Schirmer <os@emlix.com>
Signed-off-by: Alan Cox <alan@linux.intel.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>