net/mlx5e: Determine source port properly for vlan push action

Termination tables are used for vlan push actions on uplink ports.
To support RoCE dual port the source port value was placed in a register.
Fix the code to use an API method returning the source port according to
the FW capabilities.

Fixes: 10caabda ("net/mlx5e: Use termination table for VLAN push actions")
Signed-off-by: Dmytro Linkin <dmitrolin@mellanox.com>
Reviewed-by: Jianbo Liu <jianbol@mellanox.com>
Reviewed-by: Oz Shlomo <ozsh@mellanox.com>
Reviewed-by: Roi Dayan <roid@mellanox.com>
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>