ath9k: fix tx99 with monitor mode interface

Tx99 is typically configured via a monitor mode interface, which does
not get added to the driver as a vif. Since the code currently expects
a configured virtual interface for tx99, enabling tx99 via debugfs fails.
Since the vif is not needed anyway, remove all checks for it.

Signed-off-by: Felix Fietkau <nbd@nbd.name>
[kvalo@codeaurora.org: s/CPTCFG/CONFIG/]
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>