staging: mt7621-dma: add SPDX GPL-2.0+ license identifier

Add SPDX GPL-2.0+ license to fix missing SPDX warning
 reported by checkpatch.pl.
Signed-off-by: Jules Irenge <jbi.octave@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/staging/mt7621-dma/mtk-hsdma.c

+// SPDX-License-Identifier: GPL-2.0+
 /*
  *  Copyright (C) 2015, Michael Lee <igvtee@gmail.com>
  *  MTK HSDMA support