net/mlx5e: vxlan: Return bool instead of opaque ptr in port_lookup()

struct mlx5_vxlan_port is not exposed to the outside callers, it is
redundant to return a pointer to it from mlx5_vxlan_port_lookup(), to be
only used as a boolean, so just return a boolean.

Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>