netdev-windows: Fix bugs in flag update and MAC address copy functions.

The .update_flags function in netdev-windows was dummy. But we need to
return the existing flags for link status to be shown as up in the
confdb.

There was a bug in copying the MAC address.

We fix these two issues in this patch.

Signed-off-by: Nithin Raju <nithin@vmware.com>
Co-Authored-by: Ankur Sharma <ankursharma@vmware.com>
Signed-off-by: Ankur Sharma <ankursharma@vmware.com>
Acked-by: Alin Gabriel Serdean <aserdean@cloudbasesolutions.com>
Tested-by: Alin Gabriel Serdean <aserdean@cloudbasesolutions.com>
Signed-off-by: Ben Pfaff <blp@nicira.com>