ASoC: tfa9879: clean up bindings

Fix a couple of nitpicks:

- list #sound-dai-cells as a required property.
- The chip supports full speed I2C; don't indicate standard mode only.
- status = "okay" is just noise.
- The chip is an amplifier, not a codec.
- consistently indent with tabs.

Signed-off-by: Peter Rosin <peda@axentia.se>
Signed-off-by: Mark Brown <broonie@kernel.org>