Address remaining comments from original driver patch series

* Move RD_FIFO_CFG to be ordered corretly
* Expand spinlock comment
Signed-off-by: Ryan Case <ryandcase@chromium.org>
Reviewed-by: Stephen Boyd <swboyd@chromium.org>
Reviewed-by: Douglas Anderson <dianders@chromium.org>
Signed-off-by: Mark Brown <broonie@kernel.org>