net: mscc: ocelot: rename aggr_count to num_ports_in_lag

It makes it a bit easier to read and understand the code that deals with
balancing the 16 aggregation codes among the ports in a certain LAG.

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