serial: imx: Add braces to avoid ambiguous 'else'

Commit 17b8f2a3 ("serial: imx: add support for half duplex rs485")
introduced the following build warning:

drivers/tty/serial/imx.c: In function 'imx_set_termios':
drivers/tty/serial/imx.c:1301:7: warning: suggest explicit braces to avoid ambiguous 'else' [-Wparentheses]

Add the suggested braces to avoid the build warning.

Signed-off-by: Fabio Estevam <fabio.estevam@freescale.com>
Acked-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>