serial: imx: Do not store/restore the UBRC register

UBRC is a read-only register, so we should not store and restore it inside
imx_flush_buffer().

Reported-by: Fugang Duan <B38611@freescale.com>
Signed-off-by: Fabio Estevam <fabio.estevam@freescale.com>
Acked-by: Fugang Duan <B38611@freescale.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>