iwlwifi: mvm: allow a debug knob for Tx A-MSDU even if rate control forbids it

There is a debugfs knob to configure the maximal length
of the A-MSDU. If this value is not 0 (which is the
default), allow Tx A-MSDU even if the rate control
disallows it.
While at it, add "unlikely" to the if that limits the
length of the A-MSDU based on the debugfs hook.

Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>