staging: comedi: usbduxsigma: use attach_usb() hook

Change the usbduxsigma driver to use the new attach_usb() hook in struct
comedi_driver to auto-configure probed USB devices after the firmware is
loaded.

Move the release of the driver's static 'start_stop_sem' semaphore in
usbduxsigma_attach() to occur a bit later for convenience, otherwise the
new usbduxsigma_attach_common() would need to be split in two.  I don't
think the slight delay in releasing the semaphore matters too much; it's
only used in the USB probe and disconnect functions and when attaching
and detaching comedi devices.

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