dmaengine: nbpfaxi: Mark expected switch fall-through

In preparation to enabling -Wimplicit-fallthrough, mark switch cases
where we are expecting to fall through.
Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>
Signed-off-by: Vinod Koul <vkoul@kernel.org>

drivers/dma/nbpfaxi.c

@@ -479,6 +479,7 @@ static size_t nbpf_xfer_size(struct nbpf_device *nbpf,
 
 	default:
 		pr_warn("%s(): invalid bus width %u\n", __func__, width);
+		/* fall through */
 	case DMA_SLAVE_BUSWIDTH_1_BYTE:
 		size = burst;
 	}