mac80211: disable power save when scanning

When software scanning we need to disable power save so that all possible
probe responses and beacons are received. For hardware scanning assume that
hardware will take care of that and document that assumption.

Signed-off-by: Kalle Valo <kalle.valo@nokia.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>