AP: Fix HT 20/40 co-ex transition timer cancellation on iface removal

AP: Fix HT 20/40 co-ex transition timer cancellation on iface removal

When removing an interface, hostapd_bss_deinit() frees all associated
STAs. If any of the stations is 40MHz intolerant, the cleanup invokes
ht40_intolerant_remove(), that in turn registers a 20->40MHz transition
timer for the last station (ap_ht2040_timeout() function). That timer is
never canceled; once it executes, the interface is gone, most likely
resulting in a segfault when referencing it.

While hostapd_interface_deinit() cancels the transition timer, it does
so before cleaning up STAs. Move the cancellation after STA cleanup to
cancel any timer that was registered during that operation.

Signed-off-by: Martin Willi <martin@strongswan.org>