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

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: 175ad2ec89fe ("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@changeid
Signed-off-by: Johannes Berg <johannes.berg@intel.com>