dmaengine: shdmac: fixup WARNING of slave caps retrieval

ecc19d17
(dmaengine: Add a warning for drivers not using the generic slave
caps retrieval) added WARN() for DMA_SLAVE.
Kernel will shows WARNING without this patch.

Signed-off-by: Kuninori Morimoto <kuninori.morimoto.gx@renesas.com>
Acked-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Tested-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Vinod Koul <vinod.koul@intel.com>