drivers/rtc/rtc-88pm860x.c: fix rtc irq enable callback

According to 88pm860x spec, rtc alarm irq enable control is bit3 for
RTC_ALARM_EN, so fix it.

Signed-off-by: Jett.Zhou <jtzhou@marvell.com>
Cc: Axel Lin <axel.lin@gmail.com>
Cc: Haojian Zhuang <haojian.zhuang@marvell.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>