mac80211: Do not scan for IBSS merge with a fixed BSSID.

Currently, when we are on an IBSS network with no active station,
we would scan for other BSSID, even if fixed_bssid is on,  due to
a bug in ibss.c,  where fixed_channel would be checked instead of
fixed_bssid.  This would trigger useless scans where scan results
would not be used anyway.

This patch also reverts commit 39d02a7d,
which assumed that the ifibss->fixed_channel check was legitimate
to disable single-channel scans.  IBSS single-channel scan should
now be fixed.

Signed-off-by: Nicolas Cavallari <cavallar@lri.fr>
Signed-off-by: John W. Linville <linville@tuxdriver.com>