staging: comedi: mite: tidy up mite dma channel request/release

For aesthetics, make the actual "request" function static and change
mite_request_channel_in_range() into a wrapper that calls the internal
function. Change the inline function that requests any free channel
into an export that also calls the internal function.

Move the 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>