crypto: crypto4xx - get rid of redundant using_sd variable

using_sd is used as a stand-in for sa_command_0.bf.scatter
that we need to set anyway, so we might as well just prevent
double-accounting.

Signed-off-by: Christian Lamparter <chunkeey@gmail.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>