POWER: fix compile warnings in log_wakeup_reason

Change I81addaf420f1338255c5d0638b0d244a99d777d1 introduced compile
warnings, fix these.

Change-Id: I05482a5335599ab96c0a088a7d175c8d4cf1cf69
Signed-off-by: Ruchi Kandoi <kandoiruchi@google.com>