The AES engine only supports 128 and 256 bit keys so we should correctly
test for that.

Cc: Herbert Xu <herbert@gondor.apana.org.au>
Reported-by: NJoe Perches <joe@perches.com>
Signed-off-by: NJamie Iles <jamie@jamieiles.com>
Signed-off-by: NHerbert Xu <herbert@gondor.apana.org.au>