mac80211: Fixed a bug in IBSS merge

First, both beacons and probe responses can be used for IBSS merge.
Next, sdata->u.ibss.bssid was always true (and thus IBSS merge was
disabled). We should use sdata->u.ibss.fixed_bssid instead.

Signed-off-by: Benoit Papillault <benoit.papillault@free.fr>
Signed-off-by: John W. Linville <linville@tuxdriver.com>