qcom: ssr: Send sysmon shutdown command during subsystem shutdown

When the ref-count for a subsystem goes down to 0, i.e.
there are no current clients for it, the subsystem is
shutdown by calling the shutdown callbacks registered
by the subsystem driver.
Before doing so, send the sysmon shutdown command to
the subsystem so that it could take care of any cleanup
required at its end.

Change-Id: I36559de328581b9c856e042eadcf45b02e2c764b
Signed-off-by: Deepak Katragadda <dkatraga@codeaurora.org>