wifi: rtw89: chan: add sub-entity swap function to cover replacing

Originally, we replaced sub-entity of index 0 with another one in some
cases. However, we will need a swap here in following implementations.
So, we introduce it ahead and change code from replacing to swapping.

Signed-off-by: Zong-Zhe Yang <kevin_yang@realtek.com>
Signed-off-by: Ping-Ke Shih <pkshih@realtek.com>
Signed-off-by: Kalle Valo <kvalo@kernel.org>
Link: https://msgid.link/20240206030624.23382-3-pkshih@realtek.com