sh: mark DMA slave ID 0 as invalid

This makes it possible to leave DMA slave IDs in the platform data
at default 0 value without hitting DMA channel allocation error paths.

Signed-off-by: Guennadi Liakhovetski <g.liakhovetski@gmx.de>
Signed-off-by: Paul Mundt <lethal@linux-sh.org>

arch/sh/include/cpu-sh4/cpu/sh7724.h

@@ -285,6 +285,7 @@ enum {
 };
 
 enum {
+	SHDMA_SLAVE_INVALID,
 	SHDMA_SLAVE_SCIF0_TX,
 	SHDMA_SLAVE_SCIF0_RX,
 	SHDMA_SLAVE_SCIF1_TX,