staging: comedi: das800: remove forward declarations

Move the das800_cancel() and das800_interrupt() functions to remove
the need for the remaining 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>