Add Motorola emu audio dock support

Add Emu Antipop mixer control handling.
Motorola emu audio dock (USB->USB charge/audio Y cable) support
can be enabled by BOARD_USES_MOTOROLA_EMU_AUDIO.

Also add one missing QCOM_USBAUDIO_ENABLED ifdef

Change-Id: Ie67339b222252411d3c7c0aaea6493a0ff10fd18