net/mlx5: LAG, replace mlx5_get_dev_index with LAG sequence number

net/mlx5: LAG, replace mlx5_get_dev_index with LAG sequence number

Introduce mlx5_lag_get_dev_seq() which returns a device's sequence
number within the LAG: master is always 0, remaining devices numbered
sequentially. This provides a stable index for peer flow tracking and
vport ordering without depending on native_port_num.

Replace mlx5_get_dev_index() usage in en_tc.c (peer flow array
indexing) and ib_rep.c (vport index ordering) with the new API.

Signed-off-by: Shay Drory <shayd@nvidia.com>
Reviewed-by: Mark Bloch <mbloch@nvidia.com>
Signed-off-by: Tariq Toukan <tariqt@nvidia.com>
Link: https://patch.msgid.link/20260309093435.1850724-7-tariqt@nvidia.com
Signed-off-by: Leon Romanovsky <leon@kernel.org>