mlxsw: spectrum_router: Extract a helper from mlxsw_sp_port_vlan_router_join()

Split out of mlxsw_sp_port_vlan_router_join() the part that checks for RIF
and dispatches to __mlxsw_sp_port_vlan_router_join(), leaving it as wrapper
that just manages the router lock.

The new function, mlxsw_sp_port_vlan_router_join_existing(), will be useful
as an atom in later patches.

Signed-off-by: Petr Machata <petrm@nvidia.com>
Reviewed-by: Amit Cohen <amcohen@nvidia.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>