staging: comedi/drivers: use auto_attach instead of attach_usb

Change the usbdux, usbduxfast and usbduxsigma drivers to use the new
`auto_attach()` method instead of the `attach_usb()` method.  I plan to
remove the `attach_usb()` and `attach_pci()` methods from `struct
comedi_driver` once nothing is using them.

Signed-off-by: Ian Abbott <abbotti@mev.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>