staging: comedi: refactor dt2814 driver and use module_comedi_driver

Move the struct comedi_driver to the end of the source and refactor
the code to remove the forward declarations.

Convert the driver to use the module_comedi_driver() macro which
makes the code smaller and a bit simpler.

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>