vdpa/mlx5: Retrieve BAR address suitable any function

struct mlx5_core_dev has a bar_addr field that contains the correct bar
address for the function regardless of whether it is pci function or sub
function. Use it.

Fixes: 1958fc2f

 ("net/mlx5: SF, Add auxiliary device driver")
Signed-off-by: Eli Cohen <elic@nvidia.com>
Reviewed-by: Parav Pandit <parav@nvidia.com>
Link: https://lore.kernel.org/r/20210408091047.4269-4-elic@nvidia.com
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Acked-by: Jason Wang <jasowang@redhat.com>