audio/hal: Set playback buffer sizes to integral multiple msec

- The call to pcm_write was taking varying time to complete.
  This was because DSP always expects the buffer duration to
  be an integral multiple msec. When this is not the case,
  DSP waits for the rest of the data to be filled too.
  This accumalates the delay and causes the variation
  in timing.
- Change the deep buffer playback buffer size to 960 samples(20msec)
  and low latency to 240 samples (5msec) to fix the issue.

Change-Id: I9448920e89595a65cf92a5abd9187e02043b699a