use caf audio variant

Who cares what it's called. This just means less patching outside
device tree. This is misleading because it would indicate we're
building something from hardware/qcom/audio-caf which isn't the
case. Audio HAL in device tree of course has always been CAF based.

Change-Id: I13f19ed3e34ef5e6c4ee5176bb2fc806b3f0cd29