crypto: cfb - convert to skcipher_alloc_instance_simple()

The CFB template just wraps a single block cipher algorithm, so simplify
it by converting it to use skcipher_alloc_instance_simple().

Cc: James Bottomley <James.Bottomley@HansenPartnership.com>
Signed-off-by: Eric Biggers <ebiggers@google.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>