staging: comedi: move detach out of post-config

`comedi_device_postconfig()` calls `comedi_device_detach()` on failure.
Remove that call and make the callers of `comedi_device_postconfig()`
call `comedi_device_detach()` themselves if it returns an error.  This
seems more logical as the callers of `comedi_device_postconfig()` called
`comedi_device_detach()` anyway if they didn't call
`comedi_device_postconfig()`.

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