usb-serial: possible irq lock inversion (PPP vs. usb/serial)

=========================================================
[ INFO: possible irq lock inversion dependency detected ]
diff --git a/drivers/usb/serial/generic.c b/drivers/usb/serial/generic.c
index 1727135..21cbaa0 100644
--- a/drivers/usb/serial/generic.c
+++ b/drivers/usb/serial/generic.c
@@ -175,14 +175,14 @@
 
 	/* only do something if we have a bulk out endpoint */
 	if (serial->num_bulk_out) {
-		spin_lock(&port->lock);
+		spin_lock_bh(&port->lock);
 		if (port->write_urb_busy) {
-			spin_unlock(&port->lock);
+			spin_unlock_bh(&port->lock);
 			dbg("%s - already writing", __FUNCTION__);
 			return 0;
 		}
 		port->write_urb_busy = 1;
-		spin_unlock(&port->lock);
+		spin_unlock_bh(&port->lock);
 
 		count = (count > port->bulk_out_size) ? port->bulk_out_size : count;