CRAS: audio_thread - Apply input dsp to amount of data in buffer.
CRAS: audio_thread - Apply input dsp to amount of data in buffer.

The number of frames that have been written to the buffer is the
correct amount to pass through the DSP.  Using anything else can break
when there are multiple input streams with different buffer sizes
leading to partial writes.

BUG=chromium:238971
TEST=web audio input tests, listen for breakups.

Change-Id: I3e50fb06aba2b0921950b5d54bf3a1edafc16e0b
Signed-off-by: Dylan Reid <c791bfe44996c1eed9c7149a0c5f12a86d58eb85@chromium.org>
Reviewed-on: https://gerrit.chromium.org/gerrit/50949

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