mac80211: fix rhashtable conversion

My conversion of the mac80211 station hash table to rhashtable
completely broke the lookup in sta_info_get() as it no longer
took into account the virtual interface. Fix that.

Fixes: 7bedd0cf

 ("mac80211: use rhashtable for station table")
Signed-off-by: Johannes Berg <johannes.berg@intel.com>