crypto: img-hash - use API helpers to setup fallback request

Rather than setting up the fallback request by hand, use
ahash_request_set_callback() and ahash_request_set_crypt() API helpers
to properly setup the new request.

This also ensures that the completion callback is properly passed down
to the fallback algorithm, which avoids a crash with async fallbacks.

Signed-off-by: Ovidiu Panait <ovidiu.panait.oss@gmail.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>