mac80211: update oper_channel on ibss join

Commit 13c40c54 ("mac80211: Add HT operation modes for IBSS") broke
ibss operation by mistakenly removing the local->oper_channel
update (causing ibss to start on the wrong channel). fix it.

Signed-off-by: Eliad Peller <eliad@wizery.com>
Acked-by: Simon Wunderlich <siwu@hrz.tu-chemnitz.de>
Signed-off-by: John W. Linville <linville@tuxdriver.com>