Previously, struct wpa_group was created when the first station enters
the group and the struct wpa_group was not freed when all station left
the group. This causes a problem because wpa_group will enter
FATAL_FAILURE when a wpa_group is running while the AP_VLAN interface
has already been removed.
Fix this by adding a reference counter to struct wpa_group and free a
group if it is unused.
Signed-off-by: Michael Braun <michael-dev@fami-braun.de>