CRAS: Fix get_avail_frames to set used correctly.
CRAS: Fix get_avail_frames to set used correctly.

get_avail_frames wasn't setting used correctly if there was an error
or if the number of frames returned was greater than the buffer size,
which happens when the buffer underruns.

BUG=none
TEST=make check and run cras on Mario.

Change-Id: I77914639507fe260da46b8d9ed327cd0195fe921
Signed-off-by: Dylan Reid <c791bfe44996c1eed9c7149a0c5f12a86d58eb85@chromium.org>

drinkcat/adhd,drinkcat/adhd,drinkcat/adhd,drinkcat/adhd,drinkcat/adhd,drinkcat/adhd