Initialize iface->sta_seen on allocation

Previously, struct hostapd_iface sta_seen list head was initialized only
when completing interface setup. This left a window for operation that
could potentially iterate through the list before the list head has been
initialized. While the existing code checked iface->num_sta_seen to
avoid this case, it is much cleaner to initialize the list when struct
hostapd_iface is allocated to avoid any accidental missing of the extra
checks before list iteration.

Signed-off-by: Jouni Malinen <j@w1.fi>