net: dsa: microchip: lan937x: add dsa_tag_protocol

This patch update the ksz_get_tag_protocol to return LAN937x specific
tag if the chip id matches one of LAN937x series switch

Signed-off-by: Arun Ramadoss <arun.ramadoss@microchip.com>
Signed-off-by: David S. Miller <davem@davemloft.net>