net/mlx5e: Use generic name for the forwarding dev pointer

Rename tun_dev to fwd_dev within mlx5e_tc_update_priv struct
since future implementation may introduce other device types
which the handler is forwarding to.

Signed-off-by: Ariel Levkovich <lariel@nvidia.com>
Signed-off-by: Saeed Mahameed <saeedm@nvidia.com>