msm: bam_dmux: add downlink processing state logging

State transitions of the downlink packet processing task are not directly
logged.  This leads to information gaps when debugging downlink issues.

Add explict logging of the state transitions to provide complete
information on what the downlink processing task has done.

Change-Id: Ie253c9739447da6ed236e865b7f1ed318e13cb6e
Signed-off-by: Jeffrey Hugo <jhugo@codeaurora.org>