staging: mt7621-eth: Add SPDX license identifier

Add the SPDX GPL-2.0 license identifier to fix checkpatch.pl warning
Issue reported by checkpatch.pl warning:
"WARNING: Missing or malformed SPDX-License-Identifier tag in line 1"
Signed-off-by: Jules Irenge <jbi.octave@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/staging/mt7621-eth/ethtool.h

+/* SPDX-License-Identifier: GPL-2.0 */
 /*   This program is free software; you can redistribute it and/or modify
  *   it under the terms of the GNU General Public License as published by
  *   the Free Software Foundation; version 2 of the License