wifi: mac80211: handle tasklet frames before stopping

The code itself doesn't want to handle frames from the driver
if it's already stopped, but if the tasklet was queued before
and runs after the stop, then all bets are off. Flush queues
before actually stopping, RX should be off at this point since
all the interfaces are removed already, etc.

Reported-by:  <syzbot+8830db5d3593b5546d2e@syzkaller.appspotmail.com>
Link: https://msgid.link/20240515135318.b05f11385c9a.I41c1b33a2e1814c3a7ef352cd7f2951b91785617@changeid
Signed-off-by: Johannes Berg <johannes.berg@intel.com>