ASoC: wcd_cpe_services: Make the worker thread standalone

The worker thread can either be stopped through kthread_stop or could
return from the function on some conditions. Since thread has two exit
points, this is causing a race condition where kthread_stop indefinitely
waits for the thread to exit. Make the thread standalone and always call
do_exit itself to exit instead of stopping it through kthread_stop.

CRs-fixed: 972943
Change-Id: If95cbd6ee895d566887453e98421d1514147441b
Signed-off-by: Bhalchandra Gajare <gajare@codeaurora.org>