staging: comedi: mite: move mite_prep_dma()

For aesthetics, move this functions to a more logical spot in the
code and add a docbook comment for the exported function.

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>