ASoC: sun4i-i2s: Fix sun8i volatile regs

The FIFO TX reg is volatile and sun8i i2s register
mapping is different from sun4i.

Even if in this case it's doesn't create an issue,
Avoid setting some regs that are undefined in sun8i.

Acked-by: Maxime Ripard <mripard@kernel.org>
Reviewed-by: Chen-Yu Tsai <wens@csie.org>
Signed-off-by: Clément Péron <peron.clem@gmail.com>
Link: https://lore.kernel.org/r/20201030144648.397824-8-peron.clem@gmail.com
Signed-off-by: Mark Brown <broonie@kernel.org>