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

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[ INFO: possible irq lock inversion dependency detected ]
diff --git a/drivers/usb/serial/ir-usb.c b/drivers/usb/serial/ir-usb.c
index 1738b0b..1b348df 100644
--- a/drivers/usb/serial/ir-usb.c
+++ b/drivers/usb/serial/ir-usb.c
@@ -342,14 +342,14 @@
 	if (count == 0)
 		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);
 
 	transfer_buffer = port->write_urb->transfer_buffer;
 	transfer_size = min(count, port->bulk_out_size - 1);