ath11k: Fix beamformee STS in HE cap

STS Beamformee capability sent from firmware is overwritten as
num_tx_chains - 1.
When num_tx_chains is 2, then STS is set to invalid value 1.
Since STS is not limited by number of TX chain, not overwriting the
capability received from firmware.

Tested on: IPQ6018 WLAN.HK.2.4.0.1-00303-QCAHKSWPL_SILICONZ-1

Signed-off-by: Ramya Gnanasekar <rgnanase@codeaurora.org>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>
Link: https://lore.kernel.org/r/1604933229-15815-1-git-send-email-rgnanase@codeaurora.org