wifi: iwlwifi: mvm: refactor TX csum mode check

There are two modes now, and we have two places checking
that must be in sync. Refactor the logic into a new small
helper function.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Gregory Greenman <gregory.greenman@intel.com>
Link: https://lore.kernel.org/r/20230413102635.ef6246f4b73b.I44820ec095634dd0bba3007465cf25e4ce1c77c6@changeid
Signed-off-by: Johannes Berg <johannes.berg@intel.com>