iwm: fix regression from r365419 (ieee80211_media_change())

In r365419 ieee80211_media_change() callers were updated to not longer
act on the obselete ENETRESET return code.
While in the old days iwm has done a stop/init cycle in these cases,
this was not executed since r193340.
As a consequence simplify iwm code as well by passing ieee80211_media_change()
right to ieee80211_vap_attach() as there is no more need for a local
implementation.

The actual problem was fixed in stable/12 since r365608;  this just
reduces changes to HEAD.