crypto: ccp - Ignore tag length when decrypting GCM ciphertext

AES GCM input buffers for decryption contain AAD+CTEXT+TAG. Only
decrypt the ciphertext, and use the tag for comparison.

Fixes: 36cf515b

 ("crypto: ccp - Enable support for AES GCM on v5 CCPs")
Cc: <stable@vger.kernel.org>
Signed-off-by: Gary R Hook <gary.hook@amd.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>