mac80211: ignore (E)CSA in probe response frames

Seth reports that some APs, notably the Netgear WNDAP360, send
invalid ECSA IEs in probe response frames with the operating
class and channel number both set to zero, even when no channel
switch is being done. As a result, any scan while connected to
such an AP results in the connection being dropped.

Fix this by ignoring any channel switch announcment in probe
response frames entirely, since we're connected to the AP we
will be receiving a beacon (and maybe even an action frame) if
a channel switch is done, which is sufficient.

Cc: stable@vger.kernel.org # 3.10
Reported-by: Seth Forshee <seth.forshee@canonical.com>
Tested-by: Seth Forshee <seth.forshee@canonical.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>