staging: comedi: usbduxsigma: absorb usbduxsigma_attach_common() into caller

This function is only called by usbduxsigma_auto_attach(). Absorb it
and refactor the code a bit to match the usbdux driver.

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>