CRAS: dev_stream - Limit based on converted frames in mix.

The number of frames written should be limited based on the converted
number of frames.
When mixing a stream, there is no need to update the count.  The
minimum level has been determined before the funciton is called.

Change-Id: I0d2c223b1381b8aa0a2437e2922192f33d2d82a4
Signed-off-by: Dylan Reid <dgreid@chromium.org>