crypto: qat - do not offload zero length requests

If a zero length request is submitted through the skcipher api,
do not offload it and return success.

Reviewed-by: Conor Mcloughlin <conor.mcloughlin@intel.com>
Tested-by: Sergey Portnoy <sergey.portnoy@intel.com>
Signed-off-by: Giovanni Cabiddu <giovanni.cabiddu@intel.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>