crypto: ccp/gcm - use const time tag comparison.

commit 538a5a072e6ef04377b180ee9b3ce5bae0a85da4 upstream.

Avoid leaking GCM tag through timing side channel.

Fixes: 36cf515b ("crypto: ccp - Enable support for AES GCM on v5 CCPs")
Cc: <stable@vger.kernel.org> # v4.12+
Signed-off-by: Cfir Cohen <cfir@google.com>
Acked-by: Gary R Hook <ghook@amd.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>