staging: comedi: usbdux: remove 'ifnum' from the private data

The 'ifnum' is only used 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>