crypto: atmel - Fix authenc support when it is set to m

As it is if CONFIG_CRYPTO_DEV_ATMEL_AUTHENC is set to m it is in
effect disabled.  This patch fixes it by using IS_ENABLED instead
of ifdef.

Fixes: 89a82ef8

 ("crypto: atmel-authenc - add support to...")
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
Reviewed-by: Tudor Ambarus <tudor.ambarus@microchip.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>