crypto: qat - fix a signedness bug in get_service_enabled()

The "ret" variable needs to be signed or there is an error message which
will not be printed correctly.

Fixes: 0cec19c7 ("crypto: qat - add support for compression for 4xxx")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Acked-by: Giovanni Cabiddu <giovanni.cabiddu@intel.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>