Volume should take VOLUME_MIN_DB value when audio is muted.

Audio can be heard when muting volume as the minimum volume
value takes 0.0f instead of VOLUME_MIN_DB (-758).

This patch fixes the minimum volume value which should be
VOLUME_MIN_DB instead of 0.0f.

Change-Id: I4cb8093b6d9f18e5330ba5f770b9133baa77542f
Signed-off-by: Safa Boumaiza <Safax.boumaiza@intel.com>
Signed-off-by: Zhiquan Liu <zhiquan.liu@intel.com>