ath10k: fix beamformer VHT sounding dimensions capability

Similarly to the VHT STS, this is supposed to be propagated by firmware.
In case it's not, use the default value, but as last resort.

Signed-off-by: Bartosz Markowski <bartosz.markowski@tieto.com>
Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>