axolotl: audio: fix echo introduced by recent ACDB changes

Eliminating the Sidetone Effect in Headset Phonecall Usecases.

Note: Setting "<ctl name="IIR0 INP0 MUX" value="DEC0" />" of
"amic2" to "0" has the same effect.

Change-Id: I2f6da38f3976df4885af8a93106102fda64c2baf
Signed-off-by: Alexander Martinz <amartinz@shiftphones.com>