iwlwifi: mvm: add support for new TX CMD API

TX CMD API has changed to support offload assist.
Currently we do not enable checksum yet, but must set the
padding indication, to avoid FW errors.
Set other amsdu flag as well.
The rest of the flags will be configured only if HW csum
is enabled and will be set in future patches.
This change is backward compatible.

Signed-off-by: Sara Sharon <sara.sharon@intel.com>
Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>