hwmon: (sht15) Use devm_regulator_get_optional()

Since the sht15 driver supports operation without an external vref
regulator the driver should use the new devm_regulator_get_optional() to
indicate that a stub regulator should not be provided.

Signed-off-by: Mark Brown <broonie@linaro.org>
Acked-by: Guenter Roeck <linux@roeck-us.net>