crypto: nx - fix nx-aes-gcm verification

This patch fixes a bug in the nx-aes-gcm implementation.
Corrected the code so that the authtag is always verified after
decrypting and not just when there is associated data included.
Also, corrected the code to retrieve the input authtag from src
instead of dst.

Reviewed-by: Fionnuala Gunter <fin@linux.vnet.ibm.com>
Reviewed-by: Marcelo Cerri <mhcerri@linux.vnet.ibm.com>
Signed-off-by: Joy Latten <jmlatten@linux.vnet.ibm.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>