soc: dfc: fix use-after-free

There is a race condition when dfc_qmi_query_flow tries
to access the dfc client data while the dfc client is
exiting due to SSR, which results in a use-after-free.
To fix it, we will check a new flag dfc_client_exiting[i]
before accessing dfc_qmi_query_flow. Once the work is started,
dfc client will not be deleted until it is done and the
workqueue is flushed.

Change-Id: I76ab4a99b3f46cddd0ab1b1ba471c04417f57e94
Acked-by: Ning Cai <ncai@qti.qualcomm.com>
Signed-off-by: Conner Huff <chuff@codeaurora.org>