ASoC: pcm3168a: remove read-only status register from snd_kcontrol_new

"DACn Zero Flag" and "ADCn Overflow Flag" are read-only status
register. These information are needed for driver not for user.
Let's remove these from snd_kcontrol_new.

There is no detail explanation for these register on
datasheet, but, judging from its behavior, it seems these register
needs clock to read. Otherwise, I2C returns error.
Therefor, amixer command will be failed under non working timing.

Without this patch, user often fail amixer command.
This patch is tested on R-Car H3 ulcb-kf board, SSI3/4 TDM sound.

Signed-off-by: Kuninori Morimoto <kuninori.morimoto.gx@renesas.com>
Signed-off-by: Mark Brown <broonie@kernel.org>