usbnet: make usbnet_get_link() fall back to ethtool_op_get_link()

Make usbnet_get_link() fall back to ethtool_op_get_link() instead of
defaulting to 1.

This makes usbnet_get_link return valid results without the need for a
driver specific check_connect or mii ops as long as the driver calls
netif_carrier_{on,off}() as appropriate.  cdc_ether is an example of
such a driver.

Signed-off-by: Bjørn Mork <bjorn@mork.no>
Signed-off-by: David S. Miller <davem@davemloft.net>