Bluetooth: Let HIDP grab the device reference for connections

The core exports the hci_conn_hold_device() and hci_conn_put_device()
functions for device reference of connections. Use this to ensure that
the uevents from the parent are send after the child ones.

Based on a report by Brian Rogers <brian@xyzw.org>

Signed-off-by: Marcel Holtmann <marcel@holtmann.org>

net/bluetooth/hidp/hidp.h

@@ -126,6 +126,8 @@ int hidp_get_conninfo(struct hidp_conninfo *ci);
 struct hidp_session {
 	struct list_head list;
 
+	struct hci_conn *conn;
+
 	struct socket *ctrl_sock;
 	struct socket *intr_sock;