crypto: omap-sham - backlog handling fix

Previous commit "removed redundant locking" introduced
a bug in handling backlog.
In certain cases, when async request complete callback will
call complete() on -EINPROGRESS code, it will cause uncompleted requests.
It does not happen in implementation similar to crypto test manager,
but it will happen in implementation similar to dm-crypt.
Backlog needs to be checked before dequeuing next request.

Signed-off-by: Dmitry Kasatkin <dmitry.kasatkin@nokia.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>