cfg80211: fully initialize old channel for event

Paul reported that he got a report about undefined behaviour
that seems to me to originate in using uninitialized memory
when the channel structure here is used in the event code in
nl80211 later.

He never reported whether this fixed it, and I wasn't able
to trigger this so far, but we should do the right thing and
fully initialize the on-stack structure anyway.

Reported-by: Paul Menzel <pmenzel+linux-wireless@molgen.mpg.de>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>