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

=========================================================
[ INFO: possible irq lock inversion dependency detected ]
diff --git a/drivers/usb/serial/safe_serial.c b/drivers/usb/serial/safe_serial.c
index 789771e..1e07dfa 100644
--- a/drivers/usb/serial/safe_serial.c
+++ b/drivers/usb/serial/safe_serial.c
@@ -298,14 +298,14 @@
 		dbg ("%s - write request of 0 bytes", __FUNCTION__);
 		return (0);
 	}
-	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);
 
 	packet_length = port->bulk_out_size;	// get max packetsize