ath9k_htc: adhere to the DONT_REORDER transmit flag

Assure that frames with the fixed order flag are not reordered
relative to each other. This is accomplished by transmitting them
using a fixed priority independent of their QoS field.

Signed-off-by: Mathy Vanhoef <Mathy.Vanhoef@kuleuven.be>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>
Link: https://lore.kernel.org/r/20201104061823.197407-6-Mathy.Vanhoef@kuleuven.be