[S390] Remove BUG() statement

To avoid ugly warings for older gccs, we replace
BUG() with "return NULL", which is just as well.

Signed-off-by: Michael Holzheu <holzheu@de.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>

arch/s390/kernel/ipl.c

@@ -91,7 +91,7 @@ static char *shutdown_action_str(enum shutdown_action action)
 	case SHUTDOWN_STOP:
 		return SHUTDOWN_STOP_STR;
 	default:
-		BUG();
+		return NULL;
 	}
 }