compat: properly handle tracing

compat: properly handle tracing

We have to disable tracing on old kernels, but should
disable it only on old kernels. Since 2.6.33 it pretty
much hasn't changed at all, so we can keep it. Prior
to 2.6.33, there are various different reasons for not
supporting tracing, as noted in the tracepoint.h file.

This fixes tracing as the previous attempts here at
backporting it completely disabled it for all kernels,
not just for those where it's not possible.

Tested compilation of compat-wireless with tracing in
mac80211 against 2.6.24 through 3.3, but some versions
didn't compile due to unrelated issues.

Cc: stable@orbit-lab.org
Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Luis R. Rodriguez <mcgrof@frijolero.org>