rtc: vr41xx: make alarms useful

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.

Keep the alarms functional on character device release. Note that the PIE
are never enabled and would anyway be disabled by the core.

Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>