ASoC: rk3308_codec: using default gains on startup

We don't need to set gains when enable/disable ADC
and DAC, keep gains at last time.

And add hpout fade-in when enable DAC, and clean up
a bit of coding style for gains.

Change-Id: I2d8cdc1b0d87b55714177a1208a8f7a4e655a732
Signed-off-by: Xing Zheng <zhengxing@rock-chips.com>

Author: acgzx