net: dsa: microchip: ksz8795: use phy_port_cnt where possible

The driver is currently hard coded to SWITCH_PORT_NUM being
(TOTAL_PORT_NUM - 1) which is limited to 4 user ports for the ksz8795.
The phy_port_cnt is referring to its user ports. The patch removes the
extra define and use the assigned variable phy_port_cnt instead so the
driver can be used on different switches.

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>