crypto: atmel-aes - fix atmel-ctr-aes driver for RFC 3686

crypto_rfc3686_alloc() in crypto/ctr.c expects to be used with a stream
cipher (alg->cra_blocksize == 1).

Signed-off-by: Cyrille Pitchen <cyrille.pitchen@atmel.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>