ath10k: fix low TX rates when IBSS and HT

This fix TX problem when IBSS used in HT mode.
Before we used 6Mbps all the time for TX direction.

Reported-by: Yeoh Chun-Yeow <yeohchunyeow@gmail.com>
Signed-off-by: Janusz Dziedzic <janusz.dziedzic@tieto.com>
Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>