alsa_sound: Add support for custom LPA buffer size

 * On msm8930, during playback over A2DP with DSP Manager effects enabled,
   there's a lot of stuttering with the default buffer size. Stock kernel
   used a buffer size of 32, but that caused no audio at all in CM because
   of mismatch in buffer size between kernel and user space. So up until
   now I've been using a buffer size of 256 to get audio working at all,
   but unfortunately it causes stuttering as mentioned. Hence add support
   for a custom buffer size so we can revert to stock value used in kernel,
   and enjoy no stuttering.

Change-Id: I29113cee46d0adcc72a8c9c830e4c15d9fdd43dc