msm: pil-gss: Request the AXI port to halt before shutting down peripheral

This makes sure the bus port is idle prior to shutting down GSS, which
avoid the possibility of locking up the system fabric if GSS is shutdown
before it is idle.

Change-Id: Idf09482ebb3f7a8977ea244d6501f62b4adebbd1
Signed-off-by: Matt Wagantall <mattw@codeaurora.org>