USB: serial: Fix memory leak in sierra_release()

I found a memory leak in sierra_release() (well sierra_probe() I guess)
that looses 8 bytes each time the driver releases a device.
Signed-off-by: Len Sorensen <lsorense@csclub.uwaterloo.ca>
Acked-by: Johan Hovold <jhovold@gmail.com>
Cc: stable <stable@vger.kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/usb/serial/sierra.c

@@ -959,6 +959,7 @@ static void sierra_release(struct usb_serial *serial)
 			continue;
 		kfree(portdata);
 	}
+	kfree(serial->private);
 }
 
 #ifdef CONFIG_PM