P2P: Fix wpas_remove_persistent_peer() to use P2P mgmt interface

P2P: Fix wpas_remove_persistent_peer() to use P2P mgmt interface

The function used wpa_s->parent->conf to iterate the P2P networks and
update the configuration file. However, wpa_s->parent is not
necessarily the interface used to manage the P2P Device operations.

Fix this by accessing the configuration file of the interface
initialized to manage the P2P Device operations.

Signed-off-by: Ilan Peer <ilan.peer@intel.com>