hwmon: (sht15) clean-up the probe function

* Move the creation of sysfs attributes after the end of the
  initialization, and remove them in the error path.
* Release regulator in the error path.
* Add a soft reset command (need to wait 11ms before next command).

Signed-off-by: Vivien Didelot <vivien.didelot@savoirfairelinux.com>
Acked-by: Jonathan Cameron <jic23@cam.ac.uk>
Signed-off-by: Guenter Roeck <guenter.roeck@ericsson.com>