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

=========================================================
[ INFO: possible irq lock inversion dependency detected ]
diff --git a/drivers/usb/serial/omninet.c b/drivers/usb/serial/omninet.c
index e49f409..a764ff4 100644
--- a/drivers/usb/serial/omninet.c
+++ b/drivers/usb/serial/omninet.c
@@ -256,14 +256,14 @@
 		return (0);
 	}
 
-	spin_lock(&wport->lock);
+	spin_lock_bh(&wport->lock);
 	if (wport->write_urb_busy) {
-		spin_unlock(&wport->lock);
+		spin_unlock_bh(&wport->lock);
 		dbg("%s - already writing", __FUNCTION__);
 		return 0;
 	}
 	wport->write_urb_busy = 1;
-	spin_unlock(&wport->lock);
+	spin_unlock_bh(&wport->lock);
 
 	count = (count > OMNINET_BULKOUTSIZE) ? OMNINET_BULKOUTSIZE : count;