slimbus: Decrement channel reference during reconfigure now

Slimbus clients can specify multiple add/remove channel requests
through control_ch API, followed by reconfiguration request (also
known as "commit request"). Clients may rely on the reconfiguration
request to actuate the channel activation/removal and not the
control-channel API.
So the channel reference should be decremented during reconfiguration
request, and not control_ch API.

CRs-Fixed: 389518
Change-Id: Id6ff57c6a4cad878e35004937fb08daf3b9afe7b
Signed-off-by: Sagar Dharia <sdharia@codeaurora.org>