Bluetooth: Add hci_dev_lock to get/set device flags

Adding hci_dev_lock since hci_conn_params_(lookup|add) require this
lock.

Suggested-by: Miao-chen Chou <mcchou@chromium.org>
Signed-off-by: Abhishek Pandit-Subedi <abhishekpandit@chromium.org>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>