net: dsa: ksz9477: move get_stats64 to ksz_common.c

net: dsa: ksz9477: move get_stats64 to ksz_common.c

The mib counters for the ksz9477 is same for the ksz9477 switch and
LAN937x switch. Hence moving it to ksz_common.c file in order to have it
generic function. The DSA hook get_stats64 now can call ksz_get_stats64.

Signed-off-by: Arun Ramadoss <arun.ramadoss@microchip.com>
Reviewed-by: Oleksij Rempel <o.rempel@pengutronix.de>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Reviewed-by: Vladimir Oltean <olteanv@gmail.com>
Link: https://lore.kernel.org/r/20220426091048.9311-1-arun.ramadoss@microchip.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>