ASoC: rockchip-i2s: fix infinite loop in rockchip_snd_txctrl

We can get into an infinite loop if the I2S_CLR register fails to
clear due to a missing break statement, so add that.

Signed-off-by: Sonny Rao <sonnyrao@chromium.org>
Signed-off-by: Mark Brown <broonie@kernel.org>