dmaengine: rcar-dmac: unmap slave resource when channel is freed

The slave mapping should be removed together with other channel
resources when the channel is freed. If it's not unmapped it will hang
around forever after the channel is freed.

Fixes: 9f878603

 ("dmaengine: rcar-dmac: add iommu support for slave transfers")
Reported-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Niklas Söderlund <niklas.soderlund+renesas@ragnatech.se>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Vinod Koul <vinod.koul@intel.com>