staging: comedi: usbduxfast: remove some unused defines

Now that the private data is kzalloc'ed when the device is attached,
the NUMUSBDUXFAST define is not used. Remove it.

The SUBDEV_AD define is only used when setting up the subdevice.
Remove it and just open-code the value.

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>