staging: comedi: s626: rename struct buffer_dma

Re-tag `struct buffer_dma` to `struct s626_buffer_dma` to avoid
potential namespace clashes.

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>