rtc: mxc: avoid disabling interrupts on device close

Currently, the IRQs are disabled when the rtc character device is closed.
This means that the device needs to stay open to get alarms while the usual
use case will open the device, set the alarm and close the device as is
done in rtcwake.

Keep the alarm functional on character device release so the platform can
actually wakeup.

Reviewed-by: Fabio Estevam <fabio.estevam@nxp.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>