net: add unknown state to sysfs NIC duplex export

Currently when the NIC duplex state is DUPLEX_UNKNOWN it is exported as
full through sysfs, this patch adds support for DUPLEX_UNKNOWN. It is
handled the same way as in ethtool.

 Signed-off-by: Nikolay Aleksandrov <naleksan@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>