rpmsg: glink: reset read/write index after ssr

While system is going down fifo index are not reset which can
cause remote subsystem to read stale index.

Now resetting the index while system is going down.

CRs-Fixed: 2543040
Change-Id: Icb4010c120f4735da206a44a4eb1bcf06ae0a1b7
Signed-off-by: Deepak Kumar Singh <deesin@codeaurora.org>