ASoC: sun4i-i2s: Fix sun8i tx channel offset mask

Although not causing any noticeable issues, the mask for the
channel offset is covering too many bits.

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>