rtc: isl1208: switch to rtc_register_device

Fix possible race condition.
It is not allowed to return with an error code after RTC is registered.

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