rtc: m41t80: Workaround broken alarm functionality

The m41t80 driver can read and set the alarm, but it doesn't
seem to have a functional alarm irq.

This causes failures when the generic core sees alarm functions,
but then cannot use them properly for things like UIE mode.

Disabling the alarm functions allows proper error reporting,
and possible fallback to emulated modes. Once someone fixes
the alarm irq functionality, this can be restored.

CC: stable@kernel.org
CC: Matt Turner <mattst88@gmail.com>
CC: Nico Macrionitis <acrux@cruxppc.org>
CC: Atsushi Nemoto <anemo@mba.ocn.ne.jp>
Reported-by: Matt Turner <mattst88@gmail.com>
Reported-by: Nico Macrionitis <acrux@cruxppc.org>
Tested-by: Nico Macrionitis <acrux@cruxppc.org>
Signed-off-by: John Stultz <john.stultz@linaro.org>