staging: comedi: usbduxsigma: always clear the usb intfdata in (*detach)

Make sure the usb intfdata is always cleared when the device is
detached.

Signed-off-by: H Hartley Sweeten <hsweeten@visionengravers.com>
Reviewed-by: Ian Abbott <abbotti@mev.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>