net/mlx5: Fix query of sd_group field

The sd_group field moved in the HW spec from the MPIR register
to the vport context.
Align the query accordingly.

Fixes: f5e95632 ("net/mlx5: Expose Management PCIe Index Register (MPIR)")
Signed-off-by: Tariq Toukan <tariqt@nvidia.com>
Signed-off-by: Saeed Mahameed <saeedm@nvidia.com>