ath10k: fix TxBF VHT capabilities for 2+ RF-chain devices

Firmware doesn't report a complete and
ready-to-use vht cap. Instead the driver is
supposed to fill in the missing bits related to
number of chains.

This effectively increases Compressed Steering
Number and Number of Sounding Dimensions in
AssocReq frames for devices with more than one RF
chain and should improve TxBF performance.

Signed-off-by: Michal Kazior <michal.kazior@tieto.com>
Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>