staging: comedi: usbduxsigma: move usb_set_intfdata(intf, NULL) to (*detach)

For aesthetic reasons, move the clearing of the interface data to
the (*detach) since its set in the (*auto_attach).

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