usb: xhci: Fix config fail of FS hub behind a HS hub with MTT

If a full speed hub connects to a high speed hub which supports MTT,
the MTT field of its slot context will be set to 1 when xHCI driver
setups an xHCI virtual device in xhci_setup_addressable_virt_dev().
Once usb core fetch its hub descriptor, and need to update the xHC's
internal data structures for the device, the HUB field of its slot
context will be set to 1 too, meanwhile MTT is also set before, this
will cause configure endpoint command fail. In the case, we should
clear MTT to 0 for full speed hub according to section 6.2.2.

This keeps in sync with Linux kernel commit:
  096b110: usb: xhci: fix config fail of FS hub behind a HS hub with MTT

Signed-off-by: Bin Meng <bmeng.cn@gmail.com>