net: dsa: tag_qca: add define for handling MIB packet

Add struct to correctly parse a mib Ethernet packet.

Signed-off-by: Ansuel Smith <ansuelsmth@gmail.com>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>