mac80211: ibss: remove unnecessary call to release channel

The ieee80211_vif_use_channel() function calls
ieee80211_vif_release_channel(), so there's no need to call it
explicitly in __ieee80211_sta_join_ibss().

Signed-off-by: Luciano Coelho <luciano.coelho@intel.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>