serial/imx: let probing fail for the dt case without a valid alias

When the uart device is instantiated by dt but dt doesn't provide an
alias then better let probing fail instead of falling back to an
unrelated device id used for the line number and no platform data.

Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Shawn Guo <shawn.guo@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>