ath10k: fix wrong initialization of struct channel

chandef is initialized with NULL and on the very next line, we are using it to
get channel, which is not correct. Channel should be initialized after
obtaining chandef.

Found by cppcheck:

ath/ath10k/mac.c:839]: (error) Possible null pointer dereference: chandef
Signed-off-by: Maninder Singh <maninder1.s@samsung.com>
Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>

drivers/net/wireless/ath/ath10k/mac.c

@@ -836,7 +836,7 @@ static inline int ath10k_vdev_setup_sync(struct ath10k *ar)
 static int ath10k_monitor_vdev_start(struct ath10k *ar, int vdev_id)
 {
 	struct cfg80211_chan_def *chandef = NULL;
-	struct ieee80211_channel *channel = chandef->chan;
+	struct ieee80211_channel *channel = NULL;
 	struct wmi_vdev_start_request_arg arg = {};
 	int ret = 0;