usb: dwc3: Update maximum_speed for SuperSpeedPlus

If the maximum_speed is not set, set it to a known value, either
SuperSpeed or SuperSpeedPlus based on the type of controller we are
using. If we are on DWC_usb31 controller, check the PHY interface to see
if it is capable of SuperSpeedPlus.

Also this check is moved after dwc3_core_init() so that we can check
dwc->revision.

Signed-off-by: John Youn <johnyoun@synopsys.com>
Signed-off-by: Felipe Balbi <balbi@kernel.org>