rtc-at91rm9200: avoid spurious irqs

rtc-at91rm9200: avoid spurious irqs

commit 449321b39f6c6ebfa15d6da24f134240bd51db29 upstream

This fixes kernel http://bugzilla.kernel.org/show_bug.cgi?id=11112 (bogus
RTC update IRQs reported) for rtc-at91rm9200 by scrubbing old IRQ status
before enabling IRQs.

It also removes nonfunctional periodic IRQ support from this driver;
only update IRQs are reported, or provided by the hardware.

I suspect some other RTCs probably have versions of #11112; it's easy to
overlook, since most non-RTC drivers don't care about spurious IRQs:
they're not reported to userspace.

Signed-off-by: David Brownell <dbrownell@users.sourceforge.net>
Report-by: W Unruh <unruh@physics.ubc.ca>
Cc: Andrew Victor <avictor.za@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>