usbnet: Set link down initially for drivers that update link state

Some usbnet drivers update link state while others do not due to
hardware limitations.  Add a flag to distinguish those that do, and
set the link down initially for their devices.

This is intended to fix this bug: http://bugs.debian.org/444043

Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
Signed-off-by: David S. Miller <davem@davemloft.net>