mac80211: avoid calling useless channel context code

ieee80211_assign_chanctx() checks if local->use_chanctx is true, so
the two code block related to ieee80211_assign_chanctx() can be moved
into above if clause, emphasize that these code are executed only if
local->use_chanctx is true.

Signed-off-by: Zhao, Gang <gamerh2o@gmail.com>
[change subject]
Signed-off-by: Johannes Berg <johannes.berg@intel.com>