wifi: rtw89: correct unit for port offset and refine macro

Strictly speaking, the unit of the offset should be TU instead of ms.
So, correct it and the macro for calculation. Then, to make the macro
generic, the factor n is moved outside.

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://lore.kernel.org/r/20230119062453.58341-2-pkshih@realtek.com