staging: comedi: pcmmio: use comedi_legacy_detach() for (*detach)

The (*detach) of this driver just calls comedi_legacy_detach(). Use that
directly for the (*detach).

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>