mailbox: imx: Clear the right interrupts at shutdown

Make sure to only clear enabled interrupts keeping count
of the connection type.

Suggested-by: Oleksij Rempel <o.rempel@pengutronix.de>
Signed-off-by: Daniel Baluta <daniel.baluta@nxp.com>
Signed-off-by: Richard Zhu <hongxing.zhu@nxp.com>
Reviewed-by: Dong Aisheng <aisheng.dong@nxp.com>
Signed-off-by: Jassi Brar <jaswinder.singh@linaro.org>