slimbus: Don't allow a grouped channel to be controlled individually

Once client defines a group of channels, it should be only allowed to
be controlled as a group and not individually to avoid synchronization
issues between grouped channels.

Change-Id: Ib318497b2666eb5134a1cb220e5df6569c9fcc2e
Signed-off-by: Sagar Dharia <sdharia@codeaurora.org>