staging: comedi: amplc_dio200: dio200_common_attach() return 0

Change the successful return value of `dio200_common_attach()` from 1 to
0.  This is propagated as the return value from the driver's "attach"
(`dio200_attach()`) or "auto_attach" (`dio200_auto_attach()`) handler.
Any non-negative value will do, but 0 is more conventional than 1.

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