sound: m4: merge msm8930_htc.c into board-m4-audio.c

* Follows the same practice as the HTC s4 series and other HTC msm8930s.
* Keeps HTC hacks out of the kernel's sound core.
* Keeps HTC things in the HTC subfolder of arch/arm/mach-msm/

Change-Id: I0a2f23e2bdb966e001aba9f04d369d74378af305