iwlwifi: mvm: rs: refactor rate scale action decision

Extract the scale action decision to a different function
in preparation of modifying it. While at it also convert
the scale action values from hardcoded values to a clear enum.

Signed-off-by: Eyal Shapira <eyal@wizery.com>
Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>