ftdi_sio: fix kref leak

Commit 4a90f09b20f4622dcbff1f0e1e6bae1704f8ad8c added kref stuff to
ftdi_sio, but missed tty_kref_put at one exit point in
ftdi_process_read.

Signed-off-by: Jim Paris <jim@jtan.com>
Signed-off-by: Alan Cox <alan@redhat.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

drivers/usb/serial/ftdi_sio.c

@@ -2030,7 +2030,7 @@ static void ftdi_process_read(struct work_struct *work)
 			spin_unlock_irqrestore(&priv->rx_lock, flags);
 			dbg("%s - deferring remainder until unthrottled",
 					__func__);
-			return;
+			goto out;
 		}
 		spin_unlock_irqrestore(&priv->rx_lock, flags);
 		/* if the port is closed stop trying to read */