ASoC: sgtl5000: read chip revision for once

Store chip revision in struct sgtl5000_priv when sgtl5000_i2c_probe()
reads it out from register, so that we can use it in
sgtl5000_enable_regulators() with no need to read register again.

Signed-off-by: Shawn Guo <shawn.guo@linaro.org>
Signed-off-by: Mark Brown <broonie@linaro.org>