wifi: rtw89: add to fill TX descriptor for firmware command v2

This kind of TX descriptor is used to download firmware or send firmware
command. Because we want to reduce descriptor overhead and this only needs
two fields 'size' and 'type', hardware designers choose short form of
RX descriptor for it.

Signed-off-by: Ping-Ke Shih <pkshih@realtek.com>
Signed-off-by: Kalle Valo <kvalo@kernel.org>
Link: https://lore.kernel.org/r/20230911082049.33541-3-pkshih@realtek.com