ath9k: add back support for using active monitor interfaces for tx99

Various documented examples on how to set up tx99 with ath9k rely
on setting up a regular monitor interface for setting the channel.
My previous patch "ath9k: fix tx99 with monitor mode interface" made
it possible to set it up this way again. However, it was removing support
for using an active monitor interface, which is required for controlling
the bitrate as well, since the bitrate is not passed down with a regular
monitor interface.

This patch partially reverts the previous one, but keeps support for using
a regular monitor interface to keep documented steps working in cases
where the bitrate does not matter

Fixes: d9c52fd1

 ("ath9k: fix tx99 with monitor mode interface")
Signed-off-by: Felix Fietkau <nbd@nbd.name>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>