staging: comedi: refactor pcm3724 driver to remove forward declarations

Move the module_init/module_exit routines and the associated
struct comedi_driver and other variables to the end of the source.
This is more typical of how other drivers are written and removes
the need for the forward declarations.

Signed-off-by: H Hartley Sweeten <hsweeten@visionengravers.com>
Cc: Ian Abbott <abbotti@mev.co.uk>
Cc: Mori Hess <fmhess@users.sourceforge.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>