mailbox: imx: Disable the clock on devm_mbox_controller_register() failure

devm_mbox_controller_register() may fail, and in the case of failure the
priv->clk clock that was previously enabled, should be disabled.

Fixes: 2bb70056

 ("mailbox: Add support for i.MX messaging unit")
Signed-off-by: Fabio Estevam <festevam@gmail.com>
Reviewed-by: Peng Fan <peng.fan@nxp.com>
Acked-by: Oleksij Rempel <o.rempel@pengutronix.de>
[Jassi: fixed merge/am conflict]
Signed-off-by: Jassi Brar <jaswinder.singh@linaro.org>