net: dsa: mv88e6xxx: Refactor pause configuration

The mv88e6390 has a different mechanism for configuring pause.
Refactor the code into an ops function, and for the moment, don't add
any mv88e6390 code yet.

Signed-off-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>