iio: light: veml6070: use field to set integration time

Define the integration time within the configuration register as a field
to easy its handling as an index, preparing the driver to support
configurable integration times.

Signed-off-by: Javier Carrasco <javier.carrasco.cruz@gmail.com>
Link: https://patch.msgid.link/20241017-veml6070-integration-time-v1-2-3507d17d562a@gmail.com


Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>