iio: light: veml6030: fix IIO device retrieval from embedded device

In the Linux kernel, the following vulnerability has been resolved: iio: light: veml6030: fix IIO device retrieval from embedded device The dev pointer that is received as an argument in the in_illuminance_period_available_show function references the device embedded in the IIO device, not in the i2c client. dev_to_iio_dev() must be used to accessthe right data. The current implementation leads to a segmentation fault on every attempt to read the attribute because indio_dev gets a NULL assignment. This bug has been present since the first appearance of the driver, apparently since the last version (V6) before getting applied. A constant attribute was used until then, and the last modifications might have not been tested again.