CRAS: empty iodev - Report that cb_thresh samples are queued.
CRAS: empty iodev - Report that cb_thresh samples are queued.

Since the input and output paths have been joined in the unified_io
function, frames_queued can always return the callback threshold for a
frame count.  If used as input this is no change, when used as output,
this will indicated that more samples should be read from the attached
streams.

BUG=chromium:247160
TEST=select empty_iodev and play output to it.

Change-Id: I086e7bf5e278e1dc5affffd3c08a0cd59a84303f
Signed-off-by: Dylan Reid <c791bfe44996c1eed9c7149a0c5f12a86d58eb85@chromium.org>
Reviewed-on: https://gerrit.chromium.org/gerrit/61200
Reviewed-by: Chih-Chung Chang <ebe32a3bdce230d12f75781016b6b880113fb486@chromium.org>
Reviewed-by: Hsinyu Chao <8b91762bdbe27d3121b311091d7ca5125e9a8df6@chromium.org>

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