hwmon: (sht15) Properly handle the case CONFIG_REGULATOR=n

When CONFIG_REGULATOR isn't set, regulator_get_voltage() returns 0.
Properly handle this case by not trusting the value.

Reported-by: Jerome Oufella <jerome.oufella@savoirfairelinux.com>
Signed-off-by: Jean Delvare <khali@linux-fr.org>
Cc: Jonathan Cameron <jic23@cam.ac.uk>
Acked-by: Mark Brown <broonie@opensource.wolfsonmicro.com>
Cc: stable@kernel.org