mac80211: fix RX path

My previous patch ("mac80211: remove mixed-cell and userspace MLME code")
was too obvious to me, so obvious that a stupid bug crept in. The IBSS
RX function must be invoked for IBSS, of course, not anything != IBSS.

Reported-by: Larry Finger <Larry.Finger@lwfinger.net>
Signed-off-by: Johannes Berg <johannes@sipsolutions.net>
Tested-by: Larry Finger <Larry.Finger@lwfinger.net>
Signed-off-by: John W. Linville <linville@tuxdriver.com>