While there is no real value in this, the spec seems to mark this
attribute as mandatory from GO, so better included it regardless
of whether we have clients or not (the attribute is empty in case
no clients are connected).
"wpa_cli p2p_set peer_filter <MAC address>" can now be used to
only allow a single P2P Device (based on P2P Device Address) to be
discovered for testing. Setting the address to 00:00:00:00:00:00
disables the filter.