P2P: Set global->p2p_group_formation in wpas_p2p_join_start() for p2pdev

P2P: Set global->p2p_group_formation in wpas_p2p_join_start() for p2pdev

When a dedicated P2P device interface is used, the
global->p2p_group_formation was not set in wpas_p2p_join_start() if no
separate group interface is used. This would cause that in case of a
failure in group formation, the cleaning of p2p_in_provisioning is done
on the wrong interface. Furthermore, P2P_CANCEL command could not be
used to stop such a group-join operation. Fix this by setting the
global->p2p_group_formation correctly in case that the group interface
is reusing wpa_s->parent.

Signed-off-by: Adiel Aloni <adiel.aloni@intel.com>