usb: dwc3: workaround: metastability state on Run/Stop bit

All revisions prior to 2.20a have a known issue which
causes metastability state on Run/Stop bit if we
configure the core to work on any of the USB2-only
speeds.

The suggested workaround is just to never configure the
core to anything other than SuperSpeed.

Signed-off-by: Felipe Balbi <balbi@ti.com>