net/mlx5e: Fix error flow cleanup in mlx5e_tc_tun_create_header_ipv4/6

Be sure to release the neighbour in case of failures after successful
route lookup.

Fixes: 101f4de9

 ("net/mlx5e: Move TC tunnel offloading code to separate source file")
Signed-off-by: Eli Cohen <eli@mellanox.com>
Reviewed-by: Roi Dayan <roid@mellanox.com>
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>