iio: light: cm32181: Make lux_per_bit and lux_per_bit_base_it runtime settings

Make lux_per_bit and lux_per_bit_base_it settings stored in struct
cm32181_chip instead of a hardcoded (defined) values.

This is a preparation patch for reading some ACPI tables which specify
a device specific lux_per_bit value.

Reviewed-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>