net80211: Drain ageq before cleaning it up.

The comment above ieee80211_ageq_cleanup specifically notes that the queue
is assumed to be empty, and in order to make it so, ieee80211_ageq_drain
must be used.

Submitted by: Augustin Cavalier <waddlesplash@gmail.com>
Obtained from: Haiku (dffc3e235360cd7b71261239ee8507b7d62a1471)
MFC after: 1 week