mac80211: allow null chandef in tracing

In TDLS channel-switch operations the chandef can sometimes be NULL.
Avoid an oops in the trace code for these cases and just print a
chandef full of zeros.

Cc: stable@vger.kernel.org
Fixes: a7a6bdd0

 ("mac80211: introduce TDLS channel switch ops")
Signed-off-by: Arik Nemtsov <arikx.nemtsov@intel.com>
Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>