ASoC: max98090: make REVISION_ID readable

The REVISION_ID register is not currently marked readable. snd_soc_read()
refuses to read the register, and hence probe() fails.

Fixes: d4807ad2c4c0 ("regmap: Check readable regs in _regmap_read")
[exposed the bug, by checking for readability]
Fixes: 685e42154dcf ("ASoC: Replace max98090 Device Driver")
[left out this register from the readable list]
Signed-off-by: Stephen Warren <swarren@nvidia.com>
Signed-off-by: Mark Brown <broonie@linaro.org>

sound/soc/codecs/max98090.c

@@ -336,6 +336,7 @@ static bool max98090_readable_register(struct device *dev, unsigned int reg)
 	case M98090_REG_RECORD_TDM_SLOT:
 	case M98090_REG_SAMPLE_RATE:
 	case M98090_REG_DMIC34_BIQUAD_BASE ... M98090_REG_DMIC34_BIQUAD_BASE + 0x0E:
+	case M98090_REG_REVISION_ID:
 		return true;
 	default:
 		return false;