Use usbnet_link_change to handle link change centrally.

Cc: Peter Korsgaard <jacmet@sunsite.dk>
Signed-off-by: NMing Lei <ming.lei@canonical.com>
Acked-by: NPeter Korsgaard <jacmet@sunsite.dk>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>