net: dsa: tag_qca: add support for handling mgmt and MIB Ethernet packet

Add connect/disconnect helper to assign private struct to the DSA switch.
Add support for Ethernet mgmt and MIB if the DSA driver provide an handler
to correctly parse and elaborate the data.

Signed-off-by: Ansuel Smith <ansuelsmth@gmail.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>