Older chips only support DSA tagging. Newer chips have both DSA and
EDSA tagging. Refactor the code by adding port functions for setting the
frame mode, egress mode, and if to forward unknown frames.

This results in the helper mv88e6xxx_6065_family() becoming unused, so
remove it.
Signed-off-by: NAndrew Lunn <andrew@lunn.ch>
v3:
Verify mandatory ops for port setup
Don't set ether type for DSA port.
Signed-off-by: NDavid S. Miller <davem@davemloft.net>