usb: xhci-mtk: rebuild the way to get bandwidth domain

Rebuild the function get_bw_index(), get the bandwidth domain
directly instead its index of domain array.

Signed-off-by: Chunfeng Yun <chunfeng.yun@mediatek.com>
Link: https://lore.kernel.org/r/de618970301702c57bd352bf87df48bc17c699dd.1615170625.git.chunfeng.yun@mediatek.com


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