staging: comedi: usbduxsigma: return failure of auto attach

The function usbduxsigma_getstatusinfo() returns a negative
value in case there has been a communication error with
the board. This should always work and if this communication
fails then there is something seriously wrong with the board.
This is now returned to the caller so that it can
terminte the auto attachement. The return command also prevents
printing out the offset value in case of a fault.

Signed-off-by: Bernd Porr <mail@berndporr.me.uk>
Reviewed-by: Ian Abbott <abbotti@mev.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>