rtc/nuc900: modify enable/disable IRQs and driver data setting location

This patch does two modifications:

(1) Adjust enable/disable IRQs location,enable it after rtc
    registration and disable it prior to unregistration.

(2) Put 'platform_set_drvdata(pdev, nuc900_rtc)' in front of rtc
    registration still be safety, though there is no need to do this, when
    I move enable irq after rtc registration, I think still put
    'platform_set_drvdata' before rtc registration that would be a good
    habit.

Signed-off-by: Wan ZongShun <mcuos.com@gmail.com>
Cc: Alessandro Zummo <a.zummo@towertech.it>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>