ASoC: sun8i-codec: Fix AIF1_MXR_SRC field names

Even though they are for the left channel mixer, they are documented as
"MXR_SRC". This matches the naming scheme used for the main DAC. The "R"
is part of the abbreviation for "mixer", not a reference to the channel.

Acked-by: Chen-Yu Tsai <wens@csie.org>
Signed-off-by: Samuel Holland <samuel@sholland.org>
Link: https://lore.kernel.org/r/20200831034852.18841-5-samuel@sholland.org


Signed-off-by: Mark Brown <broonie@kernel.org>