slim_msm: Signal client's completion for per transfer notification

Slimbus ports are connected to BAM pipes and ports inform the pipe
when descriptor's data transfer is done. Clients queue a completion
per descrptor when that descriptor is processed. This completion will
get overwritten if client queues multiple descriptors. Use
per-transfer user-data as client's completion to avoid this problem.
This also reduces controller-driver involvement in data transfer.

Change-Id: I528fa0c2358b1c172db6e65c5d23b174b2ce367b
Signed-off-by: Sagar Dharia <sdharia@codeaurora.org>