staging: comedi: s626: move struct buffer_dma

Move the declaration of `struct buffer_dma` from "s626.h" to "s626.c" as
it seems more at home there.  After this move, "s626.h" just contains
macros related to hardware registers.

Signed-off-by: Ian Abbott <abbotti@mev.co.uk>
Reviewed-by: H Hartley Sweeten <hsweeten@visionengravers.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>