Wire up the iwm_updatechan and iwx_updatechan callbacks.

Wire up the iwm_updatechan and iwx_updatechan callbacks.

These callbacks were not reachable by mistake. This change is a first step
towards preventing iwx SYSASSERT 0x20101A28 as seen by beck@ and Mikhail
when an 11ac AP switches channel width. The callbacks may still not trigger
after this change. Possibly because APs use channel switch announcements (CSA)
which we currently ignore. We only check the 11n HTOP IE for channel info.
We may eventually need to add CSA support in order to detect channel
width changes in 11ac mode.

No regressions seen by jmc@ on iwx, nor by florian@, millert@ on iwm