cfg80211: fix mic_failure tracing

tsc can be NULL (mac80211 currently always passes NULL),
resulting in NULL-dereference. check before copying it.

Cc: stable@vger.kernel.org
Signed-off-by: Eliad Peller <eliadx.peller@intel.com>
Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>