usb: dwc3: core: Properly default unspecified speed

If the maximum_speed is not specified, default the device speed base on
its HW capability. Don't prematurely check HW capability before
validating the maximum_speed device property. The device property takes
precedence in dwc->maximum_speed.

Fixes: 0e1e5c47 ("usb: dwc3: add support for USB 2.0-only core configuration")
Reported-by: Chunfeng Yun <chunfeng.yun@mediatek.com>
Signed-off-by: Thinh Nguyen <thinhn@synopsys.com>
Signed-off-by: Felipe Balbi <balbi@kernel.org>