[POWERPC] powermac: disallow pmu sleep notifiers from aborting sleep

Tracing through the code, no current PMU sleep notifier can abort sleep.
Since no new PMU sleep notifiers should be added, this patch simplifies the
code and removes the ability to abort sleep.

Signed-off-by: Johannes Berg <johannes@sipsolutions.net>
Acked-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Paul Mackerras <paulus@samba.org>