ath10k: fix cleanup in ath10k_thermal_unregister

First remove the 'cooling_device#n' syslink created
for ath10k and then unregsiter from the thermal subsystem(cooling)

Signed-off-by: Mohammed Shafi Shajakhan <mohammed@qti.qualcomm.com>
Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>