hwmon: (sht15) Fix memory leak if regulator_enable() fails

Commit 3e78080f8148 ('hwmon: (sht15) Check return value of
regulator_enable()') depends on the use of devm_kmalloc() for automatic
resource cleanup in the failure cases, which was introduced in 3.7.  In
older stable branches, explicit cleanup is needed.

Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>