net: dsa: mv88e6xxx: remove link polling

The link status is polled by the generic phy layer, there's no need to
duplicate that polling with additional polling.  This additional polling
adds additional MDIO traffic, and races with the generic phy layer,
resulting in missing or duplicated link status messages.

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