staging: comedi: usbdux: tidy up usbdux_detach()

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

Refactor the code to remove an indent level.

Always unlink the urbs 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>