Use 'GPL-2.0-only' as the SPDX-License-Identifier tag

s/SPDX-License-Identifier: GPL-2.0/SPDX-License-Identifier: GPL-2.0-only/

Change-Id: I0558c468c25d8229af2c8caade25c458875913f4
Signed-off-by: Bryan Huntsman <bryanh@codeaurora.org>