serial: mxs-auart: Check for platform_get_irq() error

We should check whether platform_get_irq() failed, and in the case of error
this needs to be propagated.

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