wifi: rtw89: Add extra TX headroom for USB

In the case of USB the TX descriptor is transmitted in the same buffer
as the 802.11 frame, so add the required headroom.

Signed-off-by: Bitterblue Smith <rtl8821cerfe2@gmail.com>
Acked-by: Ping-Ke Shih <pkshih@realtek.com>
Signed-off-by: Ping-Ke Shih <pkshih@realtek.com>
Link: https://patch.msgid.link/91fb7579-60e5-4a06-8007-3312639beea6@gmail.com