cfg80211: fix and clean up cfg80211_gen_new_bssid()

Fix cfg80211_gen_new_bssid() to not rely on u64 modulo arithmetic,
which isn't needed since we really just want to mask there. Also,
clean it up to calculate the mask only once and use GENMASK_ULL()
instead of open-coding the mask calculation.
Signed-off-by: Johannes Berg <johannes.berg@intel.com>

include/net/cfg80211.h

@@ -5475,22 +5475,20 @@ cfg80211_inform_bss_frame(struct wiphy *wiphy,
  * @bssid: transmitter BSSID
  * @max_bssid: max BSSID indicator, taken from Multiple BSSID element
  * @mbssid_index: BSSID index, taken from Multiple BSSID index element
- * @new_bssid_addr: address of the resulting BSSID
+ * @new_bssid: calculated nontransmitted BSSID
  */
 static inline void cfg80211_gen_new_bssid(const u8 *bssid, u8 max_bssid,
-					  u8 mbssid_index, u8 *new_bssid_addr)
+					  u8 mbssid_index, u8 *new_bssid)
 {
-	u64 bssid_tmp, new_bssid;
-	u64 lsb_n;
+	u64 bssid_u64 = ether_addr_to_u64(bssid);
+	u64 mask = GENMASK_ULL(max_bssid - 1, 0);
+	u64 new_bssid_u64;
 
-	bssid_tmp = ether_addr_to_u64(bssid);
+	new_bssid_u64 = bssid_u64 & ~mask;
 
-	lsb_n = bssid_tmp & ((1 << max_bssid) - 1);
-	new_bssid = bssid_tmp;
-	new_bssid &= ~((1 << max_bssid) - 1);
-	new_bssid |= (lsb_n + mbssid_index) % (1 << max_bssid);
+	new_bssid_u64 |= ((bssid_u64 & mask) + mbssid_index) & mask;
 
-	u64_to_ether_addr(new_bssid, new_bssid_addr);
+	u64_to_ether_addr(new_bssid_u64, new_bssid);
 }
 
 /**