net: mscc: ocelot: set up logical port IDs centrally

The setup of logical port IDs is done in two places: from the inconclusively
named ocelot_setup_lag and from ocelot_port_lag_leave, a function that
also calls ocelot_setup_lag (which apparently does an incomplete setup
of the LAG).

To improve this situation, we can rename ocelot_setup_lag into
ocelot_setup_logical_port_ids, and drop the "lag" argument. It will now
set up the logical port IDs of all switch ports, which may be just
slightly more inefficient but more maintainable.

Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>