wifi: mac80211: limit A-MSDU subframes for client too

In AP/mesh where the stations are added by userspace, we
limit the number of A-MSDU subframes according to the
extended capabilities.

Refactor the code and extend that also to client-side.

Fixes: 506bcfa8 ("mac80211: limit the A-MSDU Tx based on peer's capabilities")
Signed-off-by: Johannes Berg <johannes.berg@intel.com>