wifi: iwlwifi: mvm: refactor TX rate handling

Refactor the injection and other frame TX rate handling
to always return the injection rate directly, by factoring
the legay rate portion out into a new function called in
the two relevant places (injection and non-injection).

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Gregory Greenman <gregory.greenman@intel.com>
Link: https://lore.kernel.org/r/20230921110727.dc920357bad0.I5ee8512fb63f0423c1da35b59fea8811d60c1ad3@changeid


Signed-off-by: Johannes Berg <johannes.berg@intel.com>