iio: light: stk3310: add chip id for STK3311-X variant

The STK3311 chip has a variant called STK3311-X, which has a different
chip id of 0x12.

Add the chip id to the driver.

Signed-off-by: Icenowy Zheng <icenowy@aosc.io>
Signed-off-by: Ondrej Jirman <megous@megous.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>