hwmon: (sht15) avoid CamelCase

This patch renames the supply_uV* variables to supply_uv* to avoid
CamelCase as warned by the checkpatch.pl script.

Signed-off-by: Vivien Didelot <vivien.didelot@savoirfairelinux.com>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>