crypto: mv_cesa - refactor copy_src_to_buf()

The main goal was to have it not do anything when a zero len parameter
was being passed (which could lead to a null pointer dereference, as in
this case p->src_sg is null, either). Using the min() macro, the lower
part of the loop gets simpler, too.

Signed-off-by: Phil Sutter <phil.sutter@viprinet.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>