soundwire: Initialize completion for defer messages

Deferred messages are async messages used to synchronize
transitions mostly while doing a bank switch on multi links.
On successful transitions these messages are marked complete
and thereby confirming that all the buses performed bank switch
successfully.

So, initialize the completion structure for the same.

Signed-off-by: Sanyog Kale <sanyog.r.kale@intel.com>
Signed-off-by: Shreyas NC <shreyas.nc@intel.com>
Acked-by: Pierre-Louis Bossart <pierre-louis.bossart@linux.intel.com>
Signed-off-by: Vinod Koul <vkoul@kernel.org>