ath5k: use tracing for packet tx/rx dump

This adds a few tracepoints to ath5k driver transmit and
receive callbacks in order to record packet traffic.
We record the entire packet in the trace buffer so that
the data can be extracted with trace-cmd and external
plugins.

Compared to the previous debugging calls, this approach
removes an out-of-line function call from the tx and rx
paths in the compiled-in-but-disabled case, while
improving the ability to process the logged data.

A new option, CONFIG_ATH5K_TRACER, is added so that one
may disable the tracepoints completely.

Signed-off-by: Bob Copeland <me@bobcopeland.com>
Acked-by: Bruno Randolf <br1@einfach.org>
Acked-by: Nick Kossifidis <mickflemm@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>