ASoC: amd: added byte count register offset variables to rtd

Added byte count register offset variables to audio_substream_data
structure. Modified dma pointer callback.

Signed-off-by: Vijendar Mukunda <Vijendar.Mukunda@amd.com>
Reviewed-by: Daniel Kurtz <djkurtz@chromium.org>
Signed-off-by: Mark Brown <broonie@kernel.org>