staging: comedi: usbduxfast: use a local var for the usbduxfastsub_s

Add a local variable to hold the pointer to the place in the
usbduxfast array the is being configured for the device.

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>