mlxsw: spectrum: router: Export the mlxsw_sp_router_port function

In Spectrum hardware, the router port is a virtual port that is the gateway
to the routing mechanism. Hence, in order for a packet to be L3 forwarded,
it must first be L2 forwarded to the router port inside the hardware.

Further patches in this patchset are going to introduce support in bridge
device used as an mrouter port. In this case, the router port index will be
needed in order to update the MDB entries to include the router port. Thus,
export the mlxsw_sp_router_port function, which returns the index of the
Spectrum router port.

Signed-off-by: Yotam Gigi <yotamg@mellanox.com>
Reviewed-by: Nogah Frankel <nogahf@mellanox.com>
Signed-off-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>