staging: comedi: drivers: return '0' for successful attach

The comedi core expects the driver attach functions to return a
negative errno for failure. Any other value indicates success.

For consistency in the drivers, make sure they all return '0' to
indicate success.

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>