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: d4807ad2 ("regmap: Check readable regs in _regmap_read")
[exposed the bug, by checking for readability]
Fixes: 685e4215 ("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>