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>