[net80211] Add a new capability flag to indicate that the stack should
do software A-MSDU encapsulation.

Right now there's AMSDU TX/RX capability bits and they're mostly
unused, however I'd like to maintain those as the general configuration,
not also "please software encap AMSDU." For platforms that can do
A-MSDU in firmware (iwn, iwm, etc) then their init paths can read
this flag to configure A-MSDU.