usb: xhci-mtk: use @tt_info to check the FS/LS device is under a HS hub

usb: xhci-mtk: use @tt_info to check the FS/LS device is under a HS hub

If the LS/FS device is connected to an external HS hub, the member
@tt_info in xhci_virt_device struct in not NULL, use it to check
whether a LS/FS device is under an exernal HS hub, then no need get
the slot context of this device.

Signed-off-by: Chunfeng Yun <chunfeng.yun@mediatek.com>
Link: https://lore.kernel.org/r/8117df52f16bd03087e486d7d740a183b6dd634a.1615170625.git.chunfeng.yun@mediatek.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>