mac80211: bracket driver tracing

Currently, driver tracing is sometimes invoked
after and sometimes before the actual driver
callback. This is fine as long as the driver
has no tracing itself, but as soon as it does
it gets confusing.

To make traces containing such information
easier to read, introduce a return tracer in
mac80211 that essentially brackets any driver
tracing, and invoke the real trace before the
driver's callback, only showing the return
value, if any, afterwards.

Since tracing records the process, there's no
problem with overlapping calls if that should
happen.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>