staging: comedi: usbduxfast: absorb usbduxfast_attach_common()

This function is only called by the (*auto_attach) to do the final
setup of the analog input subdevice.

For aesthetics absorb it into usbduxfast_auto_attach() and reorder
the initialization of the subdevice to better follow the "norm" in
comedi drivers.

Signed-off-by: H Hartley Sweeten <hsweeten@visionengravers.com>
Reviewed-by: Ian Abbott <abbotti@mev.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>