CRAS: alsa_io - convert audio thread to take iodev as arg.
CRAS: alsa_io - convert audio thread to take iodev as arg.

Take an iodev as an argument to the audio thread function.  This
prepares for moving the audio thread to the generic iodev code.

BUG=chromium-os:35272
TEST=make check

Change-Id: I0813395a341b5b248880e3dab2ad241ba57dcdf5
Signed-off-by: Dylan Reid <c791bfe44996c1eed9c7149a0c5f12a86d58eb85@chromium.org>
Reviewed-on: https://gerrit.chromium.org/gerrit/40065
Reviewed-by: Chih-Chung Chang <ebe32a3bdce230d12f75781016b6b880113fb486@chromium.org>

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