usb: xhci-mtk: check boundary before check tt

check_sch_tt() will access fs_bus_bw[] array, check boundary
firstly to avoid out-of-bounds issue.

Signed-off-by: Chunfeng Yun <chunfeng.yun@mediatek.com>
Link: https://lore.kernel.org/r/1629189389-18779-7-git-send-email-chunfeng.yun@mediatek.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>