USB: io_ti: query hardware-buffer status in chars_in_buffer

Query hardware-buffer status in chars_in_buffer should the write fifo be
empty.

This is needed to make the tty layer wait for hardware buffers to drain
on close.

Signed-off-by: Johan Hovold <jhovold@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/usb/serial/io_ti.c

@@ -2088,6 +2088,7 @@ static int edge_chars_in_buffer(struct tty_struct *tty)
 	struct edgeport_port *edge_port = usb_get_serial_port_data(port);
 	int chars = 0;
 	unsigned long flags;
+	int ret;
 
 	if (edge_port == NULL)
 		return 0;
@@ -2098,6 +2099,12 @@ static int edge_chars_in_buffer(struct tty_struct *tty)
 	chars = kfifo_len(&edge_port->write_fifo);
 	spin_unlock_irqrestore(&edge_port->ep_lock, flags);
 
+	if (!chars) {
+		ret = tx_active(edge_port);
+		if (ret > 0)
+			chars = ret;
+	}
+
 	dev_dbg(&port->dev, "%s - returns %d\n", __func__, chars);
 	return chars;
 }