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

The extra define SWITCH_COUNTER_NUM is a copy of the KSZ8795_COUNTER_NUM
define. This patch initializes reg_mib_cnt with KSZ8795_COUNTER_NUM,
makes use of reg_mib_cnt everywhere instead and removes the extra
define.

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>