mac80211: Creating new IBSS with fixed BSSID

This fixes a bug when creating a new IBSS network with a
fixed BSSID. The fixed BSSID situation is now with one of
my last patches handled in ieee80211_sta_find_ibss()
function.

It's more robust to test against
(ifsta->flags & IEEE80211_STA_PREV_BSSID_SET), because
ifsta->state is not seted right in every situation and so
the creating of the new IBSS network sometimes hangs after
the first try to scan for a network to merge.

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