staging: comedi: skel: use comedi_legacy_detach()

Update the skeleton driver to use the new comedi_legacy_detach()
helper in the (*detach) to release the I/O region. Also, update
the comment about it.

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