crypto: padlock-sha - Use core import and export for fallback

As padlock-sha is block-only, it needs to use core import and
export on the fallback.

Also call sha256_block_init instead of sha256_init although this
is harmless as sha256_init doesn't write into the partial block
area.

Fixes: 63dc06cd ("crypto: padlock-sha - Use API partial block handling")
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>