msm: subsystem_restart: Use the dedicated subsystem restart workqueue

ssr_wq was introduced to provide a dedicated execution
context for subsystem restart work items. Use it instead
of the system workqueue to run SSR work items.

Change-Id: Ia73f980f4bc26383edbce00f60182d49114b7994
Signed-off-by: Vikram Mulukutla <markivx@codeaurora.org>