staging: comedi: double unlock in usbduxsigma_attach()

There was a double unlock introduced in 60ff4610 ("staging: comedi:
usbduxsigma: use attach_usb() hook").

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>