The mono differential output for "Line Out" downmixes the stereo audio
from the mixer, instead of just taking the left channel.

Add a route from the "Right Mixer" to "Line Out Source Playback Route"
through the "Mono Differential" path, so DAPM doesn't shut down
everything if the left channel is muted.

Fixes: 0f909f98 ("ASoC: sun4i-codec: Add support for A31 Line Out
		      playback")
Signed-off-by: Chen-Yu Tsai <wens@csie.org>
Acked-by: Maxime Ripard <maxime.ripard@free-electrons.com>
Signed-off-by: Mark Brown <broonie@kernel.org>