iwlwifi: mvm: consider ieee80211 station max amsdu value

debugfs amsdu_len sets only the max_amsdu_len for ieee80211 station
so take it into consideration while getting max amsdu

Fixes: af2984e9 ("iwlwifi: mvm: add a debugfs entry to set a fixed size AMSDU for all TX packets")
Signed-off-by: Mordechay Goodstein <mordechay.goodstein@intel.com>
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>