crypto: crc32-generic - Use SPDX-License-Identifier

Use SPDX-License-Identifier: GPL-2.0-only, instead of hand writing it.

This also removes a reference to http://www.xyratex.com

 which seems to be
down.

Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>