serial: mxs-auart: put the device in the error path

The mxs_auart_probe() gets the device by the get_device().
So we should put the device in the error path to balance the
device's reference counter.

Signed-off-by: Huang Shijie <b32955@freescale.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>