serial: mxs-auart: Remove unneeded goto label

Instead of jumping to the 'out' label, let's return the error immediately, which
makes the code shorter and cleaner.

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