ASoC: rockchip: i2s: separate capture and playback

If we only clear the tx/rx state when both are disabled it is not
possible to start/stop one multiple times while the other is running.
Since the two are independently controlled, treat them as such and
remove the false dependency between capture and playback.

Signed-off-by: John Keeping <john@metanate.com>
Signed-off-by: Mark Brown <broonie@kernel.org>