mac80211: implement off-channel TX using hw r-o-c offload

When the driver has remain-on-channel offload,
implement off-channel transmission using that
primitive.

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

diff --cc net/mac80211/cfg.c
Simple merge

diff --cc net/mac80211/ieee80211_i.h
Simple merge

diff --cc net/mac80211/offchannel.c
Simple merge