atheros: use get_unaligned_le*() for bssid mask setting

Historically some macro helpers have been users for this,
AR5K_LOW_ID() and AR5K_HIGH_ID(), use upstream unaligned
helpers instead. This applid to ath5k and ar9170. ath9k
already uses this.

Worth noting is ath5k uses an ah_sta_id but that is already
the MAC address combined with the associaiton ID, ah_sta_id
is really ETH_ALEN in size.

Cc: Bob Copeland <me@bobcopeland.com>
Cc: Nick Kossifidis <mick@madwifi-project.org>
Cc: Christian Lamparter <chunkeey@web.de>
Signed-off-by: Luis R. Rodriguez <lrodriguez@atheros.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>