ath11k: fix a locking bug in ath11k_mac_op_start()

This error path leads to a Smatch warning:

	drivers/net/wireless/ath/ath11k/mac.c:4269 ath11k_mac_op_start()
	error: double unlocked '&ar->conf_mutex' (orig line 4251)

We're not holding the lock when we do the "goto err;" so it leads to a
double unlock.  The fix is to hold the lock for a little longer.

Fixes: c83c500b ("ath11k: enable idle power save mode")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
[kvalo@codeaurora.org: move also rcu_assign_pointer() call]
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>
Link: https://lore.kernel.org/r/YBk4GoeE+yc0wlJH@mwanda

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

@@ -4248,11 +4248,6 @@ static int ath11k_mac_op_start(struct ieee80211_hw *hw)
 	/* Configure the hash seed for hash based reo dest ring selection */
 	ath11k_wmi_pdev_lro_cfg(ar, ar->pdev->pdev_id);
 
-	mutex_unlock(&ar->conf_mutex);
-
-	rcu_assign_pointer(ab->pdevs_active[ar->pdev_idx],
-			   &ab->pdevs[ar->pdev_idx]);
-
 	/* allow device to enter IMPS */
 	if (ab->hw_params.idle_ps) {
 		ret = ath11k_wmi_pdev_set_param(ar, WMI_PDEV_PARAM_IDLE_PS_CONFIG,
@@ -4262,6 +4257,12 @@ static int ath11k_mac_op_start(struct ieee80211_hw *hw)
 			goto err;
 		}
 	}
+
+	mutex_unlock(&ar->conf_mutex);
+
+	rcu_assign_pointer(ab->pdevs_active[ar->pdev_idx],
+			   &ab->pdevs[ar->pdev_idx]);
+
 	return 0;
 
 err: