net: usbnet: update addr_assign_type if appropriate

This module generates a common default address on init,
using eth_random_addr. Set addr_assign_type to let
userspace know the address is random unless it was
overridden by the minidriver.

Signed-off-by: Bjørn Mork <bjorn@mork.no>
Acked-by: Oliver Neukum <oliver@neukum.org>
Signed-off-by: David S. Miller <davem@davemloft.net>