iio: adc: ad799x: cache regulator voltages during probe

iio: adc: ad799x: cache regulator voltages during probe

Since the reference voltage for this ADC is not expected to
change at runtime, determine the active reference voltage (either VREF
or VCC) during probe() and cache it in a single variable in the state
structure.

Suggested-by: Jonathan Cameron <jic23@kernel.org>
Suggested-by: David Lechner <dlechner@baylibre.com>
Reviewed-by: David Lechner <dlechner@baylibre.com>
Signed-off-by: Archit Anant <architanant5@gmail.com>
Reviewed-by: Andy Shevchenko <andriy.shevchenko@intel.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>