staging: comedi: comedi_bond: no need to free dev->private on detach

The comedi core will free `dev->private` if it is non-NULL after calling
the "detach" handler (`bonding_detach()`), so don't bother freeing it in
`bonding_detach()`.

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