mac80211: duplicate station's MAC address for hash table

Currently, the station hash table lookup (or iteration) must
access two cachelines for each station - the one with the hash
table node, and the one with the MAC address.

Duplicate the MAC address next to the hash node to get rid of
this. Since the MAC address is static there's no consistency
problem introduced by this.

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