ath10k: disable TX_STBC for tx chainmask of 1

Disable TX_STBC for both HT and VHT if the devices tx chainmask is '1'
TX_STBC is required only for devices with tx_chainmask > 1. This fixes
a ping failure for QCA9887 (1x1) in HT/VHT mode

Signed-off-by: Mohammed Shafi Shajakhan <mohammed@qti.qualcomm.com>
Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>