ath5k: fix regression on setting bssid mask on association

There was a typo on the second bssid mask register.
This was caused by the patch titled:

"ath5k: use common curbssid, bssidmask and macaddr"
Signed-off-by: Luis R. Rodriguez <lrodriguez@atheros.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>

drivers/net/wireless/ath/ath5k/pcu.c

@@ -292,7 +292,7 @@ void ath5k_hw_set_associd(struct ath5k_hw *ah, const u8 *bssid, u16 assoc_id)
 		ath5k_hw_reg_write(ah, get_unaligned_le32(common->bssidmask),
 				   AR_BSSMSKL);
 		ath5k_hw_reg_write(ah,
-				   get_unaligned_le16(common->curbssid + 4),
+				   get_unaligned_le16(common->bssidmask + 4),
 				   AR_BSSMSKU);
 	}