ath11k: fix thermal temperature read

Fix dangling pointer in thermal temperature event which causes
incorrect temperature read.

Tested-on: IPQ8074 AHB WLAN.HK.2.4.0.1-00041-QCAHKSWPL_SILICONZ-1

Signed-off-by: Pradeep Kumar Chitrapu <pradeepc@codeaurora.org>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>
Link: https://lore.kernel.org/r/20210218182708.8844-1-pradeepc@codeaurora.org