thermal: hwmon: inline helpers when CONFIG_THERMAL_HWMON is not set

Avoid warnings like this:
thermal_hwmon.h:29:1: warning: ‘thermal_remove_hwmon_sysfs’ defined but not used [-Wunused-function]
 thermal_remove_hwmon_sysfs(struct thermal_zone_device *tz)

Fixes: 0dd88793

 ("thermal: hwmon: move hwmon support to single file")
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Eduardo Valentin <edubezval@gmail.com>