mt76: mt7915e: Add a hwmon attribute to get the actual throttle state.

The firmware-controlled actual throttle state was previously available
by reading the cooling_device, but this confused the thermal subsystem.
Add a hwmon attribute to get it instead.

Signed-off-by: Nicolas Cavallari <nicolas.cavallari@green-communications.fr>
Signed-off-by: Felix Fietkau <nbd@nbd.name>