rtw88: fix TX secondary channel offset of 40M if current bw is 20M or 40M

TX secondary channel offset is valid only if current bandwidth is 80M,
otherwise leave this value as zero. The wrong value of txsc40 causes
MAC unpredictable behavior.

Signed-off-by: Ping-Ke Shih <pkshih@realtek.com>
Signed-off-by: Yan-Hsuan Chuang <yhchuang@realtek.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>