net: dsa: mv88e6060: remove poll_link callback

As of mv88e6xxx remove the poll_link callback since the link
state change polling is now handled by the phylib.

Tested on a mv88e6060 B0 device with a TI DM816X SoC.

Suggested-by: Andrew Lunn <andrew@lunn.ch>
Acked-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: Neil Armstrong <narmstrong@baylibre.com>
Signed-off-by: David S. Miller <davem@davemloft.net>