cfg80211: ignore invalid BSSIDs when looking for BSSes

When looking for a BSS matching given parameters, ignore invalid
BSSIDs. This avoids, for example, trying to join an IBSS that has
a multicast BSSID, which isn't supported by all drivers nor is it
a valid configuration of the IBSS so better create a new one with
a correctly chosen random BSSID.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>