serial: mxc: Move cr1 and cr2 write to mxc_serial_setbrg

Control reg write should be part of setbrg for better
buadrate generation, so move cr1 and cr2 write to
mxc_serial_setbrg

Signed-off-by: Jagan Teki <jagan@amarulasolutions.com>
Reviewed-by: Simon Glass <sjg@chromium.org>