ASoC: sun4i-i2s: Add offset to RX channel select

Whilst testing the capture functionality of the i2s on the newer
SoCs it was noticed that the recording was somewhat distorted.
This was due to the offset not being set correctly on the receiver
side.

Signed-off-by: Marcus Cooper <codekipper@gmail.com>
Acked-by: Maxime Ripard <maxime.ripard@bootlin.com>
Acked-by: Chen-Yu Tsai <wens@csie.org>
Signed-off-by: Mark Brown <broonie@kernel.org>