iwlwifi: sta: set max HE max A-MPDU according to HE capa

Currently, our max tpt is limited to max HT A-MPDU for LB,
and max VHT A-MPDU for HB. Configure HE exponent value correctly to
achieve HE max A-MPDU, both on LB and HB.

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>
Link: https://lore.kernel.org/r/iwlwifi.20201107104557.4486852ebb56.I9eb0d028e31f183597fb90120e7d4ca87e0dd6cb@changeid