net: dsa: microchip: ksz8795: remove superfluous port_cnt assignment

The port_cnt assignment will be done again in the init function.
This patch removes the previous assignment in the detect function.

Reviewed-by: Andrew Lunn <andrew@lunn.ch>
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>