net: dsa: microchip: remove usage of mib_port_count

The variable mib_port_cnt has the same meaning as port_cnt.
This driver removes the extra variable and is using port_cnt
everywhere instead.

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>