crypto: mv_cesa - fix final callback not ignoring input data

Broken by commit 6ef84509 for users
passing a request with non-zero 'nbytes' field, like e.g. testmgr.

Cc: <stable@kernel.org> # 3.0+
Signed-off-by: Phil Sutter <phil.sutter@viprinet.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>