NFC: Do not return EBUSY when stopping a poll that's already stopped

We check for the polling flag before checking if the netlink PID caller
match.

Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>
diff --git a/net/nfc/netlink.c b/net/nfc/netlink.c
index 03c31db..99bc6f7 100644
--- a/net/nfc/netlink.c
+++ b/net/nfc/netlink.c
@@ -634,6 +634,15 @@
 	if (!dev)
 		return -ENODEV;
 
+	device_lock(&dev->dev);
+
+	if (!dev->polling) {
+		device_unlock(&dev->dev);
+		return -EINVAL;
+	}
+
+	device_unlock(&dev->dev);
+
 	mutex_lock(&dev->genl_data.genl_data_mutex);
 
 	if (dev->genl_data.poll_req_pid != info->snd_pid) {