usb: xhci-mtk: check clock stability of U3_MAC

This is useful to find out the root cause when the Super Speed doesn't
work. Such as when the T-PHY is switched to PCIe or SATA, and affects
Super Speed function, the check will fail.

Signed-off-by: Chunfeng Yun <chunfeng.yun@mediatek.com>
Acked-by: Mathias Nyman <mathias.nyman@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>