wifi: mac80211: fix # of MSDU in A-MSDU calculation

During my refactoring I wanted to get rid of the switch,
but replaced it with the wrong calculation. Fix that.

Fixes: 175ad2ec ("wifi: mac80211: limit A-MSDU subframes for client too")
Reported-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Gregory Greenman <gregory.greenman@intel.com>
Link: https://lore.kernel.org/r/20230827135854.51bf1b8b0adb.Iffbd337fdad2b86ae12f5a39c69fb82b517f7486@changeidSigned-off-by: Johannes Berg <johannes.berg@intel.com>

net/mac80211/sta_info.c

@@ -2986,7 +2986,7 @@ void ieee80211_sta_set_max_amsdu_subframes(struct sta_info *sta,
 				   WLAN_EXT_CAPA9_MAX_MSDU_IN_AMSDU_MSB) << 1;
 
 	if (val)
-		sta->sta.max_amsdu_subframes = 4 << val;
+		sta->sta.max_amsdu_subframes = 4 << (4 - val);
 }
 
 #ifdef CONFIG_LOCKDEP