ASoC: max98373: Changed amp shutdown register as volatile

0x20FF(amp global enable) register was defined as non-volatile,
but it is not. Overheating, overcurrent can cause amp shutdown
in hardware.
'regmap_write' compare register readback value before writing
to avoid same value writing. 'regmap_read' just read cache
not actual hardware value for the non-volatile register.
When amp is internally shutdown by some reason, next 'AMP ON'
command can be ignored because regmap think amp is already ON.
Signed-off-by: Ryan Lee <ryans.lee@maximintegrated.com>
Link: https://lore.kernel.org/r/20210325033555.29377-1-ryans.lee@maximintegrated.comSigned-off-by: Mark Brown <broonie@kernel.org>

sound/soc/codecs/max98373-i2c.c

@@ -446,6 +446,7 @@ static bool max98373_volatile_reg(struct device *dev, unsigned int reg)
 	case MAX98373_R2054_MEAS_ADC_PVDD_CH_READBACK:
 	case MAX98373_R2055_MEAS_ADC_THERM_CH_READBACK:
 	case MAX98373_R20B6_BDE_CUR_STATE_READBACK:
+	case MAX98373_R20FF_GLOBAL_SHDN:
 	case MAX98373_R21FF_REV_ID:
 		return true;
 	default:

sound/soc/codecs/max98373-sdw.c

@@ -220,6 +220,7 @@ static bool max98373_volatile_reg(struct device *dev, unsigned int reg)
 	case MAX98373_R2054_MEAS_ADC_PVDD_CH_READBACK:
 	case MAX98373_R2055_MEAS_ADC_THERM_CH_READBACK:
 	case MAX98373_R20B6_BDE_CUR_STATE_READBACK:
+	case MAX98373_R20FF_GLOBAL_SHDN:
 	case MAX98373_R21FF_REV_ID:
 	/* SoundWire Control Port Registers */
 	case MAX98373_R0040_SCP_INIT_STAT_1 ... MAX98373_R0070_SCP_FRAME_CTLR: