net: dsa: microchip: ksz8795: align port_cnt usage with other microchip drivers

The ksz8795 driver is using port_cnt differently to the other microchip
DSA drivers. It sets it to the external physical port count, than the
whole port count (including the cpu port). This patch is aligning the
variables purpose with the other microchip drivers.

Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: Michael Grzeschik <m.grzeschik@pengutronix.de>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>