rtc: isl1208: enable interrupt after context preparation

The interrupt handler got enabled very early. If the interrupt cause is
triggering immediately before the context is fully prepared. This can
lead to undefined behaviour. Therefor we move the interrupt enable code
to the end of the probe function.

Signed-off-by: Michael Grzeschik <m.grzeschik@pengutronix.de>
Signed-off-by: Denis Osterland <Denis.Osterland@diehl.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>