mac80211: do remain-on-channel while idle

The IDLE handling in HW off-channel is broken right
now since we turn off IDLE only when the off-channel
period already started. Therefore, all drivers that
use it today (only iwlwifi!) must support off-channel
while idle, so playing with idle isn't needed at all.

Off-channel in general, since it's no longer used for
authentication/association, shouldn't affect PS, so
also remove that logic.

Also document a small caveat for reporting TX status
from off-channel frames in HW remain-on-channel.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>