net: phy: dp83867: Add support for active-low LEDs

Add the led_polarity_set callback for setting LED polarity.

Signed-off-by: Alexander Stein <alexander.stein@ew.tq-group.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>