qcom: subsystem_restart: Ignore SSR requests during system reboot/shutdown

When a system reboot or shutdown is already underway, ignore SSR errors
so that the system reboot/shutdown process is uninterrupted. However, log
the SSR request so that we know that the subsystem behaved unexpectedly.
While at it, also handle the scenario when subsystem is offline.

Change-Id: I790a2990749724ecd29231a933f0aa6500d9171c
Signed-off-by: Avaneesh Kumar Dwivedi <akdwived@codeaurora.org>