glink: Proper channel closing during ssr

The channels are not closed properly during ssr.

Fix the ssr code to invoke link_down to the core for
all the transports present on that subsystem. The link_down
internally moves all the channels to terminal state
and deletes them.

Change-Id: I3cb3e908e8ecf00c362b1da84e538949a1954350
Signed-off-by: Atish Kumar Patra <apatra@codeaurora.org>