ASoC: sgtl5000: Read SGTL5000_CHIP_ID in i2c_probe()

The usual place for reading chip ID is inside i2c_probe, so move it there and
also convert it to regmap.

sgtl5000_enable_regulators() needs to read the chip revision, so keep the
revision check there.

Signed-off-by: Fabio Estevam <fabio.estevam@freescale.com>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>