wifi: rtw89: set 2TX for 1SS rate by default

To improve performance in range, for 1SS rate, transmit the same signal
on 2 antenna, which is called 2TX.

Signed-off-by: Ping-Ke Shih <pkshih@realtek.com>
Link: https://patch.msgid.link/20250321034736.6269-1-pkshih@realtek.com