crypto: padlock-aes - work around Nano CPU errata in CBC mode

Extend previous workarounds for the prefetch bug to cover CBC mode,
clean up the code a bit.

Signed-off-by: Chuck Ebbert <cebbert@redhat.com>
Acked-by: Harald Welte <HaraldWelte@viatech.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>