iwlwifi: mvm: do not override amsdu size user settings

Since AMSDUs are not de-aggregated by HW in monitor mode,
we still need the option for setting large RBs (up to 12K).

Signed-off-by: Shaul Triebitz <shaul.triebitz@intel.com>
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>