mac80211: mark local variable __maybe_unused

The 'local' variable in __ieee80211_vif_copy_chanctx_to_vlans()
is only used/needed when lockdep is compiled in, mark it as such
to avoid compile warnings in the other case.

While at it, fix some indentation where it's used.

Reviewed-by: Luciano Coelho <luciano.coelho@intel.com>
Reviewed-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>