staging: comedi: das800: move das800_attach()

The move the das800_attach() function near the comedi_driver
declaration at the end of the file. This also removes the need
for some of the forward declarations.

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>