iio: adc: ti-ads7950: switch to using devm_regulator_get_enable_read_voltage()

iio: adc: ti-ads7950: switch to using devm_regulator_get_enable_read_voltage()

The regulator is enabled for the entire time the driver is bound to the
device, and we only need to access it to fetch voltage, which can be
done at probe time.

Switch to using devm_regulator_get_enable_read_voltage() which
simplifies probing and unbinding code.

Suggested-by: David Lechner <dlechner@baylibre.com>
Reviewed-by: Bartosz Golaszewski <bartosz.golaszewski@oss.qualcomm.com>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>
Tested-by: David Lechner <dlechner@baylibre.com>
Reviewed-by: Linus Walleij <linusw@kernel.org>
Reviewed-by: Andy Shevchenko <andriy.shevchenko@intel.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>