wifi: iwlwifi: mvm: always set MH len in offload_assist

wifi: iwlwifi: mvm: always set MH len in offload_assist

Some versions of the new hardware don't have a functional
version of the new offload method, but still have stricter
checks on the MAC header (MH) length in the offload assist
word. Include that even if checksumming isn't offloaded to
hardware.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Gregory Greenman <gregory.greenman@intel.com>
Link: https://lore.kernel.org/r/20230612184434.bba713f7495a.Idbc8e3ce313b51af4060326e0191bd338b3163a2@changeid
Signed-off-by: Johannes Berg <johannes.berg@intel.com>