udevadm: after validating action, use our internal string instead of optarg

udevadm: after validating action, use our internal string instead of optarg

This doesn't really change anything, but feels nicer, since it abstracts
away what device_action_from_string()/device_action_to_string() do
internally, and always uses a normalized action string (yes, there's no
ambiguity, but it's nice to stay abstract, maybe one day there is
ambiguity around this)