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

commit 3e78080f81481aa8340374d5a37ae033c1cf4272 upstream.

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>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>