usb: xhci-mtk: fix wrong remainder of bandwidth budget

The remainder of the last bandwidth bugdget is wrong,
it's the value alloacted in last bugdget, not unused.

Reported-by: Yaqii Wu <Yaqii.Wu@mediatek.com>
Signed-off-by: Chunfeng Yun <chunfeng.yun@mediatek.com>
Link: https://lore.kernel.org/r/1617179142-2681-1-git-send-email-chunfeng.yun@mediatek.com


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>