crypto: ccp - Fix sparse, use plain integer as NULL pointer

Fix sparse warning: Using plain integer as NULL pointer. Replaces
assignment of 0 to pointer with NULL assignment.

Fixes: 200664d5 (Add Secure Encrypted Virtualization ...)
Cc: Borislav Petkov <bp@suse.de>
Cc: Herbert Xu <herbert@gondor.apana.org.au>
Cc: Gary Hook <gary.hook@amd.com>
Cc: Tom Lendacky <thomas.lendacky@amd.com>
Signed-off-by: Brijesh Singh <brijesh.singh@amd.com>
Acked-by: Gary R Hook <gary.hook@amd.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>