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>

drivers/ide/pci/sc1200.c

@@ -186,8 +186,7 @@ static void sc1200_set_dma_mode(ide_drive_t *drive, const u8 mode)
 			}
 			break;
 		default:
-			BUG();
-			break;
+			return;
 	}
 
 	if (unit == 0) {			/* are we configuring drive0? */