audio: Avoid mic mute on proxy during call

 In voice call on USB headset , mute is applied on both
 RX and TX .
 Fix is to avoid muting on out PROXY to ensure the RX data
 written to USB hal without mute.

 CRs-Fixed: 763152

Change-Id: Icb84d8c68ba15b35884806b54c3a1daeb11b2bc2