LinuxKPI: 802.11 assign an(y) early chandef

The Realtek driver assumes an early chandef to be set. At the time
of linuxkpi_ieee80211_ifattach() we do not really know one yet so
try to find the first one which is available and set that.
This prevents a NULL-deref panic.

(cherry picked from commit c5b96b3eaede01a5117975af6de3483dc43673a4)