serial: imx: use locking to stop concurrent access of UCR1

Several places are accessing the UCR1 register without locking.
This probably will cause a race issue when another thread
is accessing the same register.
Add locking to preventing concurrent access of the UCR1 register.

Signed-off-by: Jiada Wang <jiada_wang@mentor.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>