mac80211_hwsim: use dyndbg for debug messages

The mac80211_hwsim doesn't offer a way to disable the debugging output.
Unfortunately, it's pretty chatty, dumping a  lot of stuff into the message
buffer.

This patch changes it to use dyndbg for controlling the debug output.
It's disabled by default, but can be enabled by a module parameter (1),
at runtime (2) or persisted in modprobe.conf (3).

  (1) modprobe mac80211_hwsim dyndbg=+p
  (2) echo "module mac80211_hwsim +p" >/sys/kernel/debug/dynamic_debug/control
  (3) echo "options mac80211_hwsim dyndbg=+p" >>/etc/modprobe.d/my.conf
Signed-off-by: Lubomir Rintel <lkundrak@v3.sk>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>