ath10k: Support different txbf configuration schemes

qca61x4 uses the vdev param as a sole sufficient configuration
for txbf while qca99x0 enables txbf during peer assoc by
combining the vdev param value with peer assoc's vht capabilities

This patch gets the appropriate txbf configuration scheme
before passing the wmi command to enable the same in the firmware.

Signed-off-by: Vivek Natarajan <nataraja@qti.qualcomm.com>
Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>