mac80211: support hw scan while idle

Currently, mac80211 goes to idle-off before starting a scan.
However, some devices that implement hw scan might not
need going idle-off in order to perform a hw scan, and
thus saving some energy and simplifying their state machine.

(Note that this is also the case for sched scan - it
currently doesn't make mac80211 go idle-off)

Add a new flag to indicate support for hw scan while idle.

Signed-off-by: Eliad Peller <eliad@wizery.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>