ath10k: fix interpretation of cooling device state

Setting the sysfs attribute ends up configuring the duty cycle,
but the interface through which the attribute is exposed
(cooling_device) is for setting the throttle/cooling state. This
is confusing the user. Hence renaming the cooling device interfaces
for better readability.

Cc: Matthias Kaehlcke <mka@google.com>
Signed-off-by: Rajkumar Manoharan <rmanohar@qti.qualcomm.com>
Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>