hwmon: (sht15) Check return value of regulator_enable()

Not having power is a pretty serious error so check that we are able to
enable the supply and error out if we can't.

Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>
Cc: stable@vger.kernel.org #3.8+; 3.0+ will need manual backport
Signed-off-by: Guenter Roeck <linux@roeck-us.net>