staging: comedi: mite: move the mite ring functions

For aesthetics, move the functions that allocate/free and initialize
the mite dma ring. They are currently kind of scattered around the
code.

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>