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

The subdevice function pointers are only setup if the device is
attached. The 'attached' flag in the private data is unnecessary.
Remove it.

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>