Ignore channel switch event if AP interface is not yet ready

Ignore channel switch event if AP interface is not yet ready

It is apparently possible to somehow trigger the driver to report a
channel switch event during ACS operation when the interface information
is not yet complete. hapd->iface->current_mode could be NULL in that
case and that would result in process termination due to NULL pointer
dereference.

It should not really be possible to trigger a channel switch during ACS
is running (i.e., before the AP mode operation has been started), but
since that has been seen in an arbitrary test sequence with interface
start/stop operations with various parameters (both valid and invalid),
better prevent a crash here by ignoring the unexpected event instead of
trying to process it.

Signed-off-by: Jouni Malinen <jouni@codeaurora.org>