staging: mt7621-eth/ethtool.c: Correction of SPDX license identifier

This patch fixes the checkpatch.p1 warning:

WARNING: Missing or malformed SPDX-License-Identifier tag in line 1
+/*

It includes the SPDX expression for GPL-2.0 according to the text license
in the top of the C file.
Signed-off-by: Carlos Henrique Lima Melara <charlesmelara@outlook.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/staging/mt7621-eth/ethtool.c

+// 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