ath5k: set mac address in add_interface

Configure the mac address in add_interface and clear it in
remove_interface so that users can change the mac address
to something other than the one in the eeprom.  Also avoid
setting it at attach time so that we won't ack packets
until fully set up.

Signed-off-by: Bob Copeland <me@bobcopeland.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>