ASoC: AMD: Always subtract bytescount

It is always correct to subtract out the starting bytescount value.  Even
in the case of 2^64 byte rollover (292 Million Years in the future
@ 48000 Hz) the math still works out.

Signed-off-by: Daniel Kurtz <djkurtz@chromium.org>
Signed-off-by: Mark Brown <broonie@kernel.org>