iio: adc: rzg2l_adc: Set driver data before enabling runtime PM

In the Linux kernel, the following vulnerability has been resolved:

iio: adc: rzg2l_adc: Set driver data before enabling runtime PM

When stress-testing the system by repeatedly unbinding and binding the ADC device in a loop, and the ADC is a supplier for another device (e.g., a thermal hardware block that reads temperature through the ADC), it may happen that the ADC device is runtime-resumed immediately after runtime PM is enabled, triggered by its consumer. At this point, since drvdata is not yet set and the driver's runtime PM callbacks rely on it, a crash can occur. To avoid this, set drvdata just after it was allocated.