net: dsa: mv88e6xxx: Add support to enabling pause

The 6185 can enable/disable 802.3z pause be setting the MyPause bit in
the port status register. Add an op to support this.

Signed-off-by: Russell King <rmk+kernel@armlinux.org.uk>
Signed-off-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>