The structure isn't used outside of its compilation unit. Make it
static.

Cc: Guennadi Liakhovetski <g.liakhovetski+renesas@gmail.com>
Cc: Vinod Koul <vinod.koul@intel.com>
Cc: dmaengine@vger.kernel.org
Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Signed-off-by: Vinod Koul <vinod.koul@intel.com>