mac80211: ibss: fix ignored channel parameter

my earlier patch "mac80211: change IBSS channel state to chandef"
created a regression by ignoring the channel parameter in
__ieee80211_sta_join_ibss, which breaks IBSS channel selection. This
patch fixes this situation by using the right channel and adopting the
selected bandwidth mode.

Cc: stable@vger.kernel.org
Signed-off-by: Simon Wunderlich <siwu@hrz.tu-chemnitz.de>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>