serial: mxs-auart: Disable clock on error path

We should disable the previously acquired clock when enabling s->clk
fails.

Signed-off-by: Fabio Estevam <fabio.estevam@nxp.com>
Reviewed-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>