CRAS: audio_thread - fix compiler warning.
CRAS: audio_thread - fix compiler warning.

Function doesn't return, make it void.

Change-Id: Id2e5f69872e04ccd5ffae34cc5ce0ca9b671e81c
Signed-off-by: Dylan Reid <c791bfe44996c1eed9c7149a0c5f12a86d58eb85@chromium.org>
Reviewed-on: https://gerrit.chromium.org/gerrit/66151
Reviewed-by: Chih-Chung Chang <ebe32a3bdce230d12f75781016b6b880113fb486@chromium.org>

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