cfg80211: add flags for off-channel capabilities

Currently mac80211 implements these for all devices,
but given restrictions of some devices that isn't
really true, so prepare for being able to remove the
capability for some mac80211 devices.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Acked-by: Kalle Valo <kvalo@qca.qualcomm.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>