powerpc/powernv: Fix fall-through warning for Clang

Fix the following fallthrough warnings (powernv_defconfig and powerpc64):

drivers/char/powernv-op-panel.c:78:2: warning: unannotated fall-through between switch labels [-Wimplicit-fallthrough]
Reported-by: Nathan Chancellor <nathan@kernel.org>
Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>

drivers/char/powernv-op-panel.c

@@ -75,6 +75,7 @@ static int __op_panel_update_display(void)
 				rc);
 			break;
 		}
+		break;
 	case OPAL_SUCCESS:
 		break;
 	default: