ASoC: pcm3168a: Update the RST gpio handling to align with documentation

The RST (reset-gpios) is low active so the driver must handle it
accordingly.

Add comments to explain clearly how the line is used.

Signed-off-by: Peter Ujfalusi <peter.ujfalusi@ti.com>
Link: https://lore.kernel.org/r/20191120131753.6831-3-peter.ujfalusi@ti.com


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