ath5k: fix regression which triggers an SME join upon assoc

This fixes a regression introduced by patch titled:

"atheros: define shared bssidmask setting"

The register for the BSSID was exchanged for the bssid mask register.

Signed-off-by: Luis R. Rodriguez <lrodriguez@atheros.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>