[SCSI] sd: check runtime PM status in sd_shutdown

sd_shutdown is called during reboot/poweroff.
It may fail if parent device, for example, ata port, was runtime suspended.

Fix it by checking runtime PM status of sd.
Exit immediately if sd was runtime suspended already.

Acked-by: Alan Stern <stern@rowland.harvard.edu>
Signed-off-by: Lin Ming <ming.m.lin@intel.com>
Signed-off-by: Jeff Garzik <jgarzik@redhat.com>