mlxsw: spectrum_router: Extract mlxsw_sp_netdevice_ipip_can_offload()

Some of the code down the road needs this logic as well.

Signed-off-by: Petr Machata <petrm@mellanox.com>
Reviewed-by: Ido Schimmel <idosch@mellanox.com>
Signed-off-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>