hso: fix disable_net

The HSO driver incorrectly creates a serial device instead of a net
device when disable_net is set. It shouldn't create anything for the
network interface.

Signed-off-by: Filip Aben <f.aben@option.com>
Reported-by: Piotr Isajew <pki@ex.com.pl>
Reported-by: Johan Hovold <jhovold@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>