iwlwifi: mvm: add a debugfs entry to set a fixed size AMSDU for all TX packets

The current debugfs entry only limits the max AMSDU for TCP.  Add a new
debugfs entry to allow setting a fixed AMSDU size for all TX packets,
including UDP and ICMP

Signed-off-by: Mordechay Goodstein <mordechay.goodstein@intel.com>
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>