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

When a system reboot or shutdown is already underway, ignore
SSR errors so that the reboot/shutdown process is uninterrupted.
However, log the SSR request so that we know that the subsystem
behaved unexpectedly.

CRs-Fixed: 416175
Change-Id: Idb07580f15d72cc1fab6045470ad494a07907a8c
Signed-off-by: Vikram Mulukutla <markivx@codeaurora.org>