net: lan966x: hardcode the number of external ports

Instead of counting the child nodes in the device tree, hardcode the
number of ports in the driver itself.  The counting won't work at all
if an ethernet port is marked as disabled, e.g. because it is not
connected on the board at all.

It turns out that the LAN9662 and LAN9668 use the same switching IP
with the same synthesis parameters. The only difference is that the
output ports are not connected. Thus, we can just hardcode the
number of physical ports to 8.

Fixes: db8bcaad

 ("net: lan966x: add the basic lan966x driver")
Signed-off-by: Michael Walle <michael@walle.cc>
Reviewed-by: Horatiu Vultur <horatiu.vultur@microchip.com>
Link: https://lore.kernel.org/r/20220704153654.11678868

-1-michael@walle.cc
Signed-off-by: Jakub Kicinski <kuba@kernel.org>