HAL: msm89{16,74} cleanup mixer on platform deinit

Avoid leaking mixer file descriptor resources. Those leaks were breaking
the VTS tests.

Test: vts-tradefed run vts --module VtsHalAudioV2_0Target
Test: call/play music/record/video...
Bug: 36311550
Change-Id: Ibacab6a988cf8fb1541a74ed875139e23e12c8d8
Signed-off-by: Kevin Rocard <krocard@google.com>