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>