staging: comedi: comedi_bond: change return value of bonding_attach()

`bonding_attach()` is the comedi "attach" handler for the driver.  Any
non-negative return value is treated as successful, but 0 is the
preferred return value on success.

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