Avoid trying to remove keys if the station is not active in iwm firmware.

Avoid trying to remove keys if the station is not active in iwm firmware.

Same fix as applied to iwx(4) in March. Since iwm(4) does not use a task
for key installation and removal the same race might not manifest here.
However, trying to delete a key while the station is not in firmware
is wrong in any case. And this code might get moved into a task later.