net: dsa: microchip: ksz9477: use internal_phy instead of phy_port_cnt

With code refactoring was introduced new variable internal_phy. Let's
use it.

Signed-off-by: Oleksij Rempel <o.rempel@pengutronix.de>
Reviewed-by: Vladimir Oltean <olteanv@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>