net: dsa: microchip: ksz8795: really set the correct number of ports

The KSZ9477 and KSZ8795 use the port_cnt field differently: For the
KSZ9477, it includes the CPU port(s), while for the KSZ8795, it doesn't.

It would be a good cleanup to make the handling of both drivers match,
but as a first step, fix the recently broken assignment of num_ports in
the KSZ8795 driver (which completely broke probing, as the CPU port
index was always failing the num_ports check).

Fixes: af199a1a ("net: dsa: microchip: set the correct number of ports")
Signed-off-by: Matthias Schiffer <matthias.schiffer@ew.tq-group.com>
Reviewed-by: Codrin Ciubotariu <codrin.ciubotariu@microchip.com>
Signed-off-by: David S. Miller <davem@davemloft.net>