audio_route: fix crash with unsupported control types

Check if control type is supported before accessing xxx_value
fields in mixer_state structure.

Also remove unused parameter warning.

Bug: 12871072.
Change-Id: I64c7d5e16728e3502f1fcd0a40658b72ec7acbe7
Merge tag 'android-4.4.3_r1' into HEAD

Android 4.4.3 release 1