mlxsw: spectrum_router: Make mlxsw_sp_netdevice_ipip_ol_up_event() void

This function only ever returns 0, so don't pretend it returns anything
useful and just make it void.

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>