[IRDA]: Race between open and disconnect in irda-usb.

It seems to me that irda_usb_net_open() must set self->netopen
under spinlock or disconnect() may fail to kill all URBs, if it is called
while an interface is opened.

Signed-off-by: Oliver Neukum <oneukum@suse.de>
Signed-off-by: Samuel Ortiz <samuel@sortiz.org>
Signed-off-by: David S. Miller <davem@davemloft.net>