ath11k: fix uninitialized rate_idx in ath11k_dp_tx_update_txcompl()

The rate_idx which passed to ath11k_debugfs_sta_add_tx_stats() by
ath11k_dp_tx_update_txcompl() is not initialized, add initialization
for it.

Tested-on: WCN6855 hw2.0 PCI WLAN.HSP.1.1-03003-QCAHSPSWPL_V1_V2_SILICONZ_LITE-2

Fixes: 1b8bb94c ("ath11k: report tx bitrate for iw wlan station dump")
Signed-off-by: Wen Gong <quic_wgong@quicinc.com>
Signed-off-by: Kalle Valo <quic_kvalo@quicinc.com>
Link: https://lore.kernel.org/r/20220209060816.423-1-quic_wgong@quicinc.com