iio: dac: max5522: simplify probe with devm_regulator_get_enable_read_voltage()

iio: dac: max5522: simplify probe with devm_regulator_get_enable_read_voltage()

Simplify probe by using devm_regulator_get_enable_read_voltage() to
get, enable and read the regulator voltage in a single call, caching
the value at probe time.

The reference voltage for this DAC is a fixed hardware configuration
that is not expected to change at runtime, so reading it once during
probe and caching the millivolt value is sufficient.

Reviewed-by: Andy Shevchenko <andriy.shevchenko@intel.com>
Signed-off-by: Antoniu Miclaus <antoniu.miclaus@analog.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>