staging: comedi: s626: replace CamelCase struct bufferDMA

Rename `struct bufferDMA` and its members to avoid CamelCase.

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>