drivers: mfd: npm1300: Fixed race condition in event callback

If an event occurs between the status registers being read and
the event being cleared, the interrupt line will remain active.

As the interrupt is edge triggered,  all future interrupts
will being ignored.  This problem will also occur if an I2C
transation fails in the callback.

The state of the interrupt pin is now checked at the end of the
callback, and a retry is attempted if the interrupt has not
been cleared.

Signed-off-by: Andy Sinclair <andy.sinclair@nordicsemi.no>