wifi: rtw89: correct VHT TX rate on 20MHz connection

It may get wrong bitrate when connecting to AP set VHT 20MHz,
and thus we fix it to follow Wi-Fi spec.

Signed-off-by: Dian-Syuan Yang <dian_syuan0116@realtek.com>
Signed-off-by: Ping-Ke Shih <pkshih@realtek.com>
Link: https://patch.msgid.link/20240731070506.46100-6-pkshih@realtek.com