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

The 'probed' flag in the private data was used to handle the
disconnect between the usb (*probe) and the comedi_driver
(*auto_attach) so that if the comedi_driver was attached without
an associated usb_device the subdevice functions would return
-ENODEV.

Now that the usb_driver (*probe) is part of the comedi_driver
(*auto_attach), the comedi_driver can only attach if the usb
device is present.

Remove the unnecessary variable from the private data and its
uses in the driver.

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>