PM: wakeup_reasons: fix race condition

log_possible_wakeup_reason() and stop_logging_wakeup_reasons() can race, as the
latter can be called from process context, and both can run on separate cores.

Change-Id: I306441d0be46dd4fe58c55cdc162f9d61a28c27d
Signed-off-by: Iliyan Malchev <malchev@google.com>