If you are on Debian 9 and systemd is assigning it a very long name you may run into problems.
ie Reason: 3=DEAUTH_LEAVING
Normally making the systemd.link would solve this on other distribution, but the way Debian sets udev rules up it doesn't.
So to fix it run:
sudo ln -s /lib/udev/rules.d/80-net-setup-link.rules /etc/udev/rules.d/ -v
and make the file:
/etc/systemd/network/10-ath9k.link
that put into it the following:
[Match]
Driver=ath9k_htc
[Link]
Name=ath9k