crypto: mv_cesa - ensure backlog is initialised

backlog is not initialised so in the case where
cpg->eng_st != ENGINE_IDLE it is never initialised and
hence which could lead to an illegal memory dereference
in the statement:

  backlog->complete(backlog, -EINPROGRESS);

Discovered with cppcheck static analsys:
[drivers/crypto/mv_cesa.c:616]:
   (error) Uninitialized variable: backlog

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>