P2P: Use p2p_init_wpa_s for P2P command redirection over D-Bus

P2P: Use p2p_init_wpa_s for P2P command redirection over D-Bus

D-Bus used the p2p_dev member in struct wpa_supplicant to track the
interface used for P2P Device operations. However, this is not needed as
the interface used for P2P Device operations is stored in struct
wpa_global->p2p_init_wpa_s.

Replace all the redirections to wpa_s->p2p_dev with redirections to
wpa_s->global->p2p_init_wpa_s and removed this member as it is no
longer needed.

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