msm: bam_dmux: wait for DL thread to shutdown in case of SSR

In case of Subsystem Restart, recent targets require that the downlink
processing thread be signaled to shutdown eariler and that the SSR
process wait for the DL thread completely shutdown otherwise an unclocked
register access could occur which might cause a kernel panic.

CRs-Fixed: 451694
Change-Id: I9216c6006bdc8941dd3c03bd6aff14cd732540bc
Signed-off-by: Jeffrey Hugo <jhugo@codeaurora.org>
Signed-off-by: Arun Kumar Neelakantam <aneela@codeaurora.org>