net: dsa: move mib->cnt_ptr reset code to ksz_common.c

mib->cnt_ptr resetting is handled in multiple places as part of
port_init_cnt(). Hence moved mib->cnt_ptr code to ksz common layer
and removed from individual product files.

Signed-off-by: Prasanna Vengateshan <prasanna.vengateshan@microchip.com>
Signed-off-by: Arun Ramadoss <arun.ramadoss@microchip.com>
Reviewed-by: Vladimir Oltean <olteanv@gmail.com>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>