dmaengine: sh: rcar-dmac: Replace zero-length arrays with DECLARE_FLEX_ARRAY() helper

Zero-length arrays are deprecated and we are moving towards adopting
C99 flexible-array members, instead. So, replace zero-length arrays
declarations in anonymous union with the new DECLARE_FLEX_ARRAY()
helper macro.

This helper allows for flexible-array members in unions.

Link: https://github.com/KSPP/linux/issues/193
Link: https://github.com/KSPP/linux/issues/217
Link: https://gcc.gnu.org/onlinedocs/gcc/Zero-Length.html
Signed-off-by: Gustavo A. R. Silva <[e-mail]>
Reviewed-by: Kees Cook <[e-mail]>
Link: https://lore.kernel.org/r/YzIdsJqsR3LH2qEK@work
Signed-off-by: Vinod Koul <[e-mail]>

drivers/dma/sh/rcar-dmac.c

···
 	struct list_head node;
 
 	union {
-		struct rcar_dmac_desc descs[0];
-		struct rcar_dmac_xfer_chunk chunks[0];
+		DECLARE_FLEX_ARRAY(struct rcar_dmac_desc, descs);
+		DECLARE_FLEX_ARRAY(struct rcar_dmac_xfer_chunk, chunks);
 	};
 };