media: dvb-core: Convert to SPDX identifier

use SPDX-License-Identifier instead of a verbose license text
and remove verbose license text.

Link: https://lore.kernel.org/linux-media/20210916020018.8550-1-caihuoqing@baidu.com



Signed-off-by: Cai Huoqing <caihuoqing@baidu.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>