iwlwifi: mvm: isolate offload assist (checksum) calculation

Isolate the entire calculation of the offload_assist field used
for HW checksumming to the iwl_mvm_tx_csum function.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>
Link: https://lore.kernel.org/r/iwlwifi.20211210110539.796ae29c90d9.Icb2f07905bebd9ed4537ca4c453069211ea71799@changeid


Signed-off-by: Luca Coelho <luciano.coelho@intel.com>