mgmt: mcumgr: Make returning rc responses when status is OK legacy

This change changes the previous mcumgr behaviour of return result
codes when the status is 0 (OK) to being legacy behaviour, instead
it will skip the rc field for these responses. If there is only an
rc field with status 0 to return, then mcumgr will now instead just
return an empty map.

Signed-off-by: Jamie McCrae <jamie.mccrae@nordicsemi.no>