sysfs: Remove return of const string
sysfs: Remove return of const string

This prevents move operations.

Change-Id: I897e7d5e7f72076144a1a1bc15abac9f76da3aee
Signed-off-by: Brad Bishop <713d098c0be4c8fd2bf36a94cd08699466677ecd@fuzziesquirrel.com>

openbmc/phosphor-hwmon,openbmc/phosphor-hwmon