Commit 3843bfd
"serial: imx: Determine device name from device tree"
broke this driver for non-devicetree boards, since
of_alias_get may not be called with a NULL pointer
as first argument.

Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>