staging: comedi: serial2002: return 0 after successful attach

A return value of >=0 indicates a successful attach to the comedi core.
Return 0 since that is more common in the kernel.

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>