DFS: Fix hostapd operation without hw_mode driver data

DFS: Fix hostapd operation without hw_mode driver data

If DFS implementation was built in, some configurations with drivers
that do not provide mode information could end up dereferencing a NULL
pointer. Fix this by skipping DFS operations in such cases since not
having information about modes and channels means that hostapd could not
perform DFS anyway (i.e., either this is not a wireless driver or the
driver takes care of DFS internally).

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