hwmon: (sht15) Convert to use devm_ functions

Convert to use devm_ functions to reduce code size and simplify the code.

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