mac80211: ieee80211_ibss_commit() cleanup

Don't call ieee80211_sta_find_ibss() directly, like it's done in STA
mode, so that the commit() call is more harmless respectively has
less site-effects.

Signed-off-by: Alina Friedrichsen <x-alina@gmx.net>
Signed-off-by: John W. Linville <linville@tuxdriver.com>