ath9k: Move roc completion to the offchannel timer

Currently, when a roc period expires, the offchannel
timer calls ieee80211_remain_on_channel_expired(), but
the roc state is cleared only when the queued work
to switch to the operating channel gets a chance to run.

This race is a problem because mac80211 can issue a
new roc request in this window. To avoid this, handle
roc completion in the offchannel timer itself.

Signed-off-by: Sujith Manoharan <c_manoha@qca.qualcomm.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>