iwlwifi: mvm: add AMSDU flag to offload assist

Enable offload assist for AMSDU when the AMSDU present flag is set.

Fixes: a830baba9c2e ("iwlwifi: mvm: support new TX API")
Signed-off-by: Sara Sharon <sara.sharon@intel.com>
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>