hwmon: Constify str parameter of hwmon_ops->read_string

The read_string callback is supposed to retrieve a pointer to a
constant string.

Signed-off-by: Jean Delvare <jdelvare@suse.de>
Reviewed-by: Peter Huewe <peterhuewe@gmx.de>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>