ath10k: fix beamformee VHT STS capability

The VHT STS CAP shall be reported by firmware to host, like in case of
QCA99x0. For QCA6174 hw family this isn't set for some reason.
So for this particular chips, let's assume it has the ability to
support VHT NDP in up to 4 STSs (which is true by the way).

Change the published beamformee STS cap accordingly to 3 or to what
the firmware reports.

Assumption so far, it suppose to be the num_rf_chains-1, was
completely wrong.

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