iwlwifi: mvm: in VHT connection use only VHT capabilities

mac80211 limits amsdu size to the minimum of HT and VHT capabilities
but since in a VHT connection we don't transmit HT frames we can discard
HT limits.

Signed-off-by: Mordechay Goodstein <mordechay.goodstein@intel.com>
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>