serial/imx: add of_alias_get_id() reference back

As of_alias_get_id() gets fixed and ready for use, the patch adds the
of_alias_get_id() reference back to imx serial driver.

Signed-off-by: Shawn Guo <shawn.guo@linaro.org>
[grant.likely: changed pr_err() to dev_err(), dropped unnecessary else]
Signed-off-by: Grant Likely <grant.likely@secretlab.ca>