ieee80211: add "max length of AMPDU" enum for VHT

Maximum length of AMPDU that an STA can receive in VHT.
length = 2 ^ (13 + max_ampdu_length_exp) - 1.

Signed-off-by: Eran Harary <eran.harary@intel.com>
Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>