staging: comedi: skel: remove most forward declarations

Move `skel_attach()`, `skel_detach()` and `skel_driver` down the file so
that a bunch of forward declarations can be removed.

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