usb: xhci-mtk: add a function to get bandwidth boundary

This is used to simplify unit test.

Signed-off-by: Chunfeng Yun <chunfeng.yun@mediatek.com>
Link: https://lore.kernel.org/r/805b3ba66c2f02a52de4440212519aaa58463039.1615170625.git.chunfeng.yun@mediatek.com


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