mac80211: use ifmgd->bssid instead of ifmgd->associated->bssid

mac80211: use ifmgd->bssid instead of ifmgd->associated->bssid

Since we always track the BSSID there when we get associated,
these are equivalent, but ifmgd->bssid saves a dereference and
thus makes the code a bit smaller, and more readable.

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