mac80211: fix ieee80211_find_sta[_by_hw]

Both of these functions can currently return
a station pointer that, to the driver, is
invalid (in IBSS mode only) because adding
the station failed. Check for that, and also
make ieee80211_find_sta() properly use the
per interface station search.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>