ide: don't BUG() on unsupported transfer modes

Fix ide-cris, cs5530, sc1200 and sis5513 host drivers to just return instead
of OOPS-ing for unsupported modes in ->set_dma_mode methods.

Acked-by: Sergei Shtylyov <sshtylyov@ru.mvista.com>
Signed-off-by: Bartlomiej Zolnierkiewicz <bzolnier@gmail.com>
diff --git a/drivers/ide/cris/ide-cris.c b/drivers/ide/cris/ide-cris.c
index e196aef..7f5bc2e 100644
--- a/drivers/ide/cris/ide-cris.c
+++ b/drivers/ide/cris/ide-cris.c
@@ -748,8 +748,7 @@
 			hold = ATA_DMA2_HOLD;
 			break;
 		default:
-			BUG();
-			break;
+			return;
 	}
 
 	if (speed >= XFER_UDMA_0)