wifi: ath11k: fix Tx power value during active CAC

Tx power is fetched from firmware's pdev stats. However, during active
CAC, firmware does not fill the current Tx power and sends the max
initialised value filled during firmware init. If host sends this power
to user space, this is wrong since in certain situations, the Tx power
could be greater than the max allowed by the regulatory. Hence, host
should not be fetching the Tx power during an active CAC.

Fix this issue by returning -EAGAIN error so that user space knows that there's
no valid value available.

Tested-on: QCN9074 hw1.0 PCI WLAN.HK.2.7.0.1-01744-QCAHKSWPL_SILICONZ-1

Fixes: 9a2aa68a ("wifi: ath11k: add get_txpower mac ops")
Signed-off-by: Aditya Kumar Singh <quic_adisi@quicinc.com>
Acked-by: Jeff Johnson <quic_jjohnson@quicinc.com>
Signed-off-by: Kalle Valo <quic_kvalo@quicinc.com>
Link: https://lore.kernel.org/r/20230912051857.2284-4-quic_adisi@quicinc.com