ath11k: fix crash caused by NULL rx_channel

During connect and disconnect stress test, crashed happened
because ar->rx_channel is NULL. Fix it by checking whether
ar->rx_channel is NULL.

Crash stack is as below:
RIP: 0010:ath11k_dp_rx_h_ppdu+0x110/0x230 [ath11k]
[ 5028.808963]  ath11k_dp_rx_wbm_err+0x14a/0x360 [ath11k]
[ 5028.808970]  ath11k_dp_rx_process_wbm_err+0x41c/0x520 [ath11k]
[ 5028.808978]  ath11k_dp_service_srng+0x25e/0x2d0 [ath11k]
[ 5028.808982]  ath11k_pci_ext_grp_napi_poll+0x23/0x80 [ath11k_pci]
[ 5028.808986]  net_rx_action+0x27e/0x400
[ 5028.808990]  __do_softirq+0xfd/0x2bb
[ 5028.808993]  irq_exit+0xa6/0xb0
[ 5028.808995]  do_IRQ+0x56/0xe0
[ 5028.808997]  common_interrupt+0xf/0xf

Tested-on: QCA6390 hw2.0 PCI WLAN.HST.1.0.1-01740-QCAHSTSWPLZ_V2_TO_X86-1
Signed-off-by: Carl Huang <cjhuang@codeaurora.org>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>
Link: https://lore.kernel.org/r/20201211055613.9310-1-cjhuang@codeaurora.org

drivers/net/wireless/ath/ath11k/dp_rx.c

@@ -2294,6 +2294,7 @@ static void ath11k_dp_rx_h_ppdu(struct ath11k *ar, struct hal_rx_desc *rx_desc,
 {
 	u8 channel_num;
 	u32 center_freq;
+	struct ieee80211_channel *channel;
 
 	rx_status->freq = 0;
 	rx_status->rate_idx = 0;
@@ -2314,9 +2315,12 @@ static void ath11k_dp_rx_h_ppdu(struct ath11k *ar, struct hal_rx_desc *rx_desc,
 		rx_status->band = NL80211_BAND_5GHZ;
 	} else {
 		spin_lock_bh(&ar->data_lock);
-		rx_status->band = ar->rx_channel->band;
-		channel_num =
-			ieee80211_frequency_to_channel(ar->rx_channel->center_freq);
+		channel = ar->rx_channel;
+		if (channel) {
+			rx_status->band = channel->band;
+			channel_num =
+				ieee80211_frequency_to_channel(channel->center_freq);
+		}
 		spin_unlock_bh(&ar->data_lock);
 		ath11k_dbg_dump(ar->ab, ATH11K_DBG_DATA, NULL, "rx_desc: ",
 				rx_desc, sizeof(struct hal_rx_desc));