net: dsa: mv88e6xxx: 6351 family also has RGMII delays

The recent refactoring of setting the MAC configuration broke setting
of RGMII delays, via the phy-mode, on the 6351 family. Add the missing
ops to the structure.

Fixes: 7340e5ecdbb1 ("net: dsa: mv88e6xxx: setup port's MAC")
Signed-off-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>