wifi: ath12k: fix warning on DMA ring capabilities event

We are seeing below warning in both reset and suspend/resume scenarios:

[ 4153.776040] ath12k_pci 0000:04:00.0: Already processed, so ignoring dma ring caps

This is because ab->num_db_cap is not cleared in
ath12k_wmi_free_dbring_caps(), so clear it to avoid such
warnings.

Tested-on: WCN7850 hw2.0 PCI WLAN.HMT.1.0-03427-QCAHMTSWPL_V1.0_V2.0_SILICONZ-1.15378.4
Tested-on: QCN9274 hw2.0 PCI WLAN.WBE.1.0.1-00029-QCAHKSWPL_SILICONZ-1

Signed-off-by: Baochen Qiang <quic_bqiang@quicinc.com>
Signed-off-by: Kalle Valo <quic_kvalo@quicinc.com>
Link: https://msgid.link/20240412060620.27519-5-quic_bqiang@quicinc.com