staging: comedi: ni_670x: return '0' for successful attach

The comedi core expects a < 0 value during the attach to indicate
an error. The normal 'success' return for the kernel is '0' so use
that here.

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