iwlwifi: change max HT and VHT A-MPDU exponent

Add two new parameters to iwl_cfg:
max_ht_ampdu_exponent and max_vht_ampdu_exponent.
These parameters, if set, will set new values to the maximum of
HT and VHT A-MPDU exponent for the A-MPDU length exponent.

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