net/mlx5: Read num_vfs before disabling SR-IOV

mlx5_device_disable_sriov() currently reads num_vfs from the PCI core.
However when mlx5_device_disable_sriov() is executed, SR-IOV is
already disabled at the PCI level.
Due to this disable_hca() cleanup is not done during SR-IOV disable
flow.

mlx5_sriov_disable()
  pci_enable_sriov()
  mlx5_device_disable_sriov() <- num_vfs is zero here.

When SR-IOV enablement fails during mlx5_sriov_enable(), HCA's are left
in enabled stage because mlx5_device_disable_sriov() relies on num_vfs
from PCI core.

mlx5_sriov_enable()
  mlx5_device_enable_sriov()
  pci_enable_sriov() <- Fails

Hence, to overcome above issues,
(a) Read num_vfs before disabling SR-IOV and use it.
(b) Use num_vfs given when enabling sriov in error unwinding path.

Fixes: d886aba6

 ("net/mlx5: Reduce dependency on enabled_vfs counter and num_vfs")
Signed-off-by: Parav Pandit <parav@mellanox.com>
Reviewed-by: Daniel Jurgens <danielj@mellanox.com>
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>