Bluetooth: Fix advertising duplicated flags

Instances may have flags set as part of its data in which case the code
should not attempt to add it again otherwise it can cause duplication:

< HCI Command: LE Set Extended Advertising Data (0x08|0x0037) plen 35
        Handle: 0x00
        Operation: Complete extended advertising data (0x03)
        Fragment preference: Minimize fragmentation (0x01)
        Data length: 0x06
        Flags: 0x04
          BR/EDR Not Supported
        Flags: 0x06
          LE General Discoverable Mode
          BR/EDR Not Supported

Signed-off-by: Luiz Augusto von Dentz <luiz.von.dentz@intel.com>
Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>