soc: qcom: bam_dmux: Ensure subsys notification dependencies are met

The subsystem notification processing depends on some resources that are
not available until after bam_dmux probes.  Since the subsystem
notification callback is registered at init, there is a race where the
required dependencies for notification processing are not met.

Move the registration to ensure all required dependencies from probe are
met.

Change-Id: I8fef8e4b9f1dc0269bf961a80acc04880403cf5b
Signed-off-by: Jeffrey Hugo <jhugo@codeaurora.org>