thermal_hwmon: Pass the originating device down to hwmon_device_register_with_info

When registering the hwmon device, we pass NULL as the device.
While this doesn't result in any immediate breakage, it leaves
the hwmon device at the root of the virtual devices, rather than
attached to the thermal zone hierarchy.

Instead, let's pass the actual device, which is part of the
thermal_zone_device structure. This also avoids the rather
unpleasant ""NULL device *" which can be generated by dev_{err,info}
in the hwmon subsystem.

Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>
Tested-by: Enric Balletbo i Serra <enric.balletbo@collabora.com>
Signed-off-by: Eduardo Valentin <edubezval@gmail.com>