[Bluetooth] Start inquiry if cache is empty

If the inquiry cache is empty it makes no sense to return zero number of
inquiry result even if the age of cache is not outdated.

include/net/bluetooth/hci_core.h

@@ -176,6 +176,12 @@ static inline void inquiry_cache_init(struct hci_dev *hdev)
 	c->list = NULL;
 }
 
+static inline int inquiry_cache_empty(struct hci_dev *hdev)
+{
+	struct inquiry_cache *c = &hdev->inq_cache;
+	return (c->list == NULL);
+}
+
 static inline long inquiry_cache_age(struct hci_dev *hdev)
 {
 	struct inquiry_cache *c = &hdev->inq_cache;

net/bluetooth/hci_core.c

@@ -406,6 +406,7 @@ int hci_inquiry(unsigned long arg)
 
 	hci_dev_lock_bh(hdev);
 	if (inquiry_cache_age(hdev) > INQUIRY_CACHE_AGE_MAX || 
+					inquiry_cache_empty(hdev) ||
 					ir.flags & IREQ_CACHE_FLUSH) {
 		inquiry_cache_flush(hdev);
 		do_inquiry = 1;