staging: comedi: refactor dt2801 driver and use module_comedi_driver

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

Refactor some of the other functions to remove the remaining
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>