wifi: ath11k: fix warning on DMA ring capabilities event

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

[69663.691847] ath11k_pci 0000:02:00.0: Already processed, so ignoring dma ring caps

This is because ab->num_db_cap is not cleared in
ath11k_wmi_free_dbring_caps(), so clear it to avoid such
warnings.

Tested-on: WCN6855 hw2.0 PCI WLAN.HSP.1.1-03125-QCAHSPSWPL_V1_V2_SILICONZ_LITE-3.6510.30

Signed-off-by: Kalle Valo <quic_kvalo@quicinc.com>
Signed-off-by: Baochen Qiang <quic_bqiang@quicinc.com>
Link: https://msgid.link/20240221024725.10057-5-quic_bqiang@quicinc.com