crypto: artpec6 - set correct iv size for gcm(aes)

The IV size should not include the 32 bit counter. Because we had the
IV size set as 16 the transform only worked when the IV input was zero
padded.

Fixes: a21eb94f ("crypto: axis - add ARTPEC-6/7 crypto accelerator driver")
Signed-off-by: Lars Persson <larper@axis.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>