Bluetooth: Add HCI_RSSI_INVALID for unknown RSSI value

The Bluetooth core specification defines the value 127 as invalid for
RSSI values. So instead of hard coding it, lets add a constant for it.
Signed-off-by: NMarcel Holtmann <marcel@holtmann.org>
Signed-off-by: NJohan Hedberg <johan.hedberg@intel.com>

include/net/bluetooth/hci.h

@@ -412,6 +412,7 @@ enum {
 
 /* The core spec defines 127 as the "not available" value */
 #define HCI_TX_POWER_INVALID	127
+#define HCI_RSSI_INVALID	127
 
 #define HCI_ROLE_MASTER		0x00
 #define HCI_ROLE_SLAVE		0x01