mac80211: fix virtual monitor interface iteration

During channel context assignment, the interface should
be found by interface iteration, so we need to assign the
pointer before the channel context.

Reported-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>
Tested-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>