net/mlx5: Implement sriov_get_vf_total_msix/count() callbacks

The mlx5 implementation executes a firmware command on the PF to change
the configuration of the selected VF.

Link: https://lore.kernel.org/linux-pci/20210314124256.70253-5-leon@kernel.org
Acked-by: Bjorn Helgaas <bhelgaas@google.com>
Signed-off-by: Leon Romanovsky <leonro@nvidia.com>