LinuxKPI: 802.11: use cfg80211_chandef_create()

Rather than manually setting up a chandef and then effectively forcing
a memcpy, use cfg80211_chandef_create() to do the work for us entirely.
This works here as we do not store the resulting chandef separately
for other use.

While here remove a duplicate assignment in cfg80211_chandef_create().

Sponsored by: The FreeBSD Foundation
MFC after: 3 days