audio-caf: hal: bugfixes to PLATFORM_MSM8960

* audio_hw.c: this relaxation of the if clause, at least on MSM8960, actually
  breaks voice calls. This patch fixes NIGHTLIES-250

* voice_extn: since MSM8960 doesn't have an implementation for this call, it
  returns the proper error code (-ENOSYS). We now deal correctly with -ENOSYS,
  but treating it as one possible expected behaviour

Change-Id: I72c7c780e3d689b648e0a414107caec8120df381