staging: comedi: mite: move the mite dma arm/disarm/reset functions

For aesthetics, move these functions to a more logical spot in the
code and add docbook comments for the exported functions.

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>