net/mlx5e: Verify encapsulation is supported

When mlx5e_attach_encap() calls mlx5e_get_tc_tun() to get the tunnel
info data struct, check that returned value is not NULL, as would be in
the case of unsupported encapsulation.

Fixes: d386939a

 ("net/mlx5e: Rearrange tc tunnel code in a modular way")
Signed-off-by: Eli Cohen <eli@mellanox.com>
Reviewed-by: Roi Dayan <roid@mellanox.com>
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>