Revert "drivers/rtc/rtc-at91rm9200.c: use a variable for storing IMR"

This reverts commit 0ef1594c.

This patch introduced a few races which cannot be easily fixed with a
small follow-up patch. Furthermore, the SoC with the broken hardware
register, which this patch intended to add support for, can only be used
with device trees, which this driver currently does not support.

[ Here is the discussion that led to this "revert" patch:
  https://lkml.org/lkml/2013/4/3/176

 ]

Cc: stable <stable@vger.kernel.org>
Signed-off-by: Johan Hovold <jhovold@gmail.com>
Signed-off-by: Nicolas Ferre <nicolas.ferre@atmel.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>