mac80211: mesh: fix wrong mesh TTL offset calculation

mesh TTL offset in Mesh Channel Switch Parameters element depends on
not only Secondary Channel Offset element, but also affected by
HT Control field and Wide Bandwidth Channel Switch element.
So use element structure to manipulate mesh channel swich param IE
after removing its constant attribution to correct the miscalculation.

Signed-off-by: Peter Oh <peter.oh@bowerswilkins.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>