wifi: mac80211_hwsim: send NDP for link (de)activation

In hwsim we now track when we receive NDP (or other frames
carrying a PM bit) to see if a link should be active, but
then of course we also need to transmit NDPs to at least
set a link to inactive when we deacivate it. Implement that
as well as sending an NDP when we activate a link, which
allows receiving frames before transmitting any.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>