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/pci/sc1200.c b/drivers/ide/pci/sc1200.c
index 0a7b320..707d5ff 100644
--- a/drivers/ide/pci/sc1200.c
+++ b/drivers/ide/pci/sc1200.c
@@ -186,8 +186,7 @@
 			}
 			break;
 		default:
-			BUG();
-			break;
+			return;
 	}
 
 	if (unit == 0) {			/* are we configuring drive0? */