mac80211: mark expected switch fall-throughs

In preparation to enabling -Wimplicit-fallthrough, mark switch cases
where we are expecting to fall through.

Notice that in some cases I replaced "fall through on else" and
"otherwise fall through" comments with just a "fall through" comment,
which is what GCC is expecting to find.

Signed-off-by: Gustavo A. R. Silva <garsilva@embeddedor.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>

net/mac80211/iface.c

@@ -1633,7 +1633,7 @@ static void ieee80211_assign_perm_addr(struct ieee80211_local *local,
 				goto out_unlock;
 			}
 		}
-		/* otherwise fall through */
+		/* fall through */
 	default:
 		/* assign a new address if possible -- try n_addresses first */
 		for (i = 0; i < local->hw.wiphy->n_addresses; i++) {