usb: xhci-mtk: use @sch_tt to check whether need do TT schedule

It's clearer to use @sch_tt to check whether need do TT schedule,
no function is changed.

Signed-off-by: Chunfeng Yun <chunfeng.yun@mediatek.com>
Link: https://lore.kernel.org/r/324a76782ccaf857a8f01f67aee435e8ec7d0e28.1615170625.git.chunfeng.yun@mediatek.com


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