fix handling of release_action in save_power controller

Since commit da6966ce the power config form validator is actually
applied, therefore release_action is now already a valid ReleaseAction
enum value. Calling ReleaseAction.from_string() on it again results in
a ValueError.

Bug: 1080685
Change-Id: Iefda403ac457aa2321d5f9ac37a93f28eb97aec8