crypto: atmel - Fix selection of CRYPTO_AUTHENC

The following error is raised when CONFIG_CRYPTO_DEV_ATMEL_AES=y and
CONFIG_CRYPTO_DEV_ATMEL_AUTHENC=m:
drivers/crypto/atmel-aes.o: In function `atmel_aes_authenc_setkey':
atmel-aes.c:(.text+0x9bc): undefined reference to `crypto_authenc_extractkeys'
Makefile:1094: recipe for target 'vmlinux' failed

Fix it by moving the selection of CRYPTO_AUTHENC under
config CRYPTO_DEV_ATMEL_AES.

Fixes: 89a82ef8

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