LinuxKPI: skbuff: no longer use IEEE80211_DEBUG to turn on debug

A port using linux(kpi) header files but not using skbuffs is hitting
the case that it cannot find opt_wlan.h. Give up to the idea that
skbuff.h is only used by wireless drivers (or in-tree) and that
IEEE80211_DEBUG (via opt_lwan.h) could autmatically compile in debug
support.

It is likely time to add a LINUXKPI_DEBUG knob in the near future
(also for linuxkpi_debug or linuxkpi_debug_rcu).

PR: 289268
MFC after: 3 days