mac80211-hwsim: Add support for HWSIM_ATTR_DESTROY_RADIO_ON_CLOSE

Add support for new attribute HWSIM_ATTR_DESTROY_RADIO_ON_CLOSE
which can be set by the user space component. The attribute
will cause the kernel to destroy all those radios that were
created by a process if that process dies. The old behaviour
i.e., radios are persistent is still the default.

Signed-off-by: Jukka Rissanen <jukka.rissanen@linux.intel.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>