audio policy: remove log spam for voice and sco volumes

Do not return an error in checkAndSetVolume() when SCO volume is modified
while SCO is not active as AudioService will always set both voice call
and SCO volumes at the same time regardless of SCO activity.

Test: Use call screening to check downlink mute.
Change-Id: I7501b28266ec4d591378d7bc4453bb5840e03af1