staging: comedi: das800: move das800_set_frequency()

Move das800_set_frequency() to avoid the need for the forward
declaration.

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>