crypto: pcbc - convert to skcipher_alloc_instance_simple()

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

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