net: dsa: microchip: move get_strings to ksz_common

ksz8795 and ksz9477 uses the same algorithm for copying the ethtool
strings. Hence moved to ksz_common to remove the redundant code.

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