pci_driver.sriov_configure should return negative value on error and
number of enabled VFs on success. But now the driver returns 0 on
success. The sriov configure still works but will cause a warning
message:

  XX VFs requested; only 0 enabled

This patch changes the return value accordingly.

Cc: stable@vger.kernel.org
Signed-off-by: Xu Yilun <yilun.xu@intel.com>
Signed-off-by: Wu Hao <hao.wu@intel.com>
Signed-off-by: Moritz Fischer <mdf@kernel.org>