staging: comedi: usbduxfast: remove 'ifnum' from private data

The interface number is only needed during the attach of the
device. Remove it from the private data.

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>