m7-common: libaudioamp: Refactor TFA9887 code

Change-Id: I95c4c378181d45980bb059f128debd41f1590125
m7-common: tfa9887: remove play silence

* Playing silence through aplay does not seem to cause the right clock
  to be enabled.

Change-Id: Ie6f2f77693bc0b8896213f82c7a055b5be73c107