staging: comedi: gsc_hpdi: absorb setup_subdevices()

This function is only called by the (*auto_attach). Absorb it into that
function to clarify the attach and remove the need for some of the forward
declarations.

For aesthetics, add some whitespace to the subdevice init.

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>