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

The variable mib_cnt is assigned with TOTAL_SWITCH_COUNTER_NUM. This
value can also be derived from the array size of mib_names. This patch
uses this calculated value instead, removes the extra define and uses
mib_cnt everywhere possible instead of the static define
TOTAL_SWITCH_COUNTER_NUM. Keeping it in a separate variable instead of
using ARRAY_SIZE everywhere instead makes the driver more flexible for
future use of devices with different amount of counters.

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>