usb: dwc3: core: Print warning on unsupported speed

The user may have more information to override the HW parameter to
specify the maximum_speed. However, if the user specifies a
maximum_speed that the controller doesn't support, print out a warning.

Signed-off-by: Thinh Nguyen <thinhn@synopsys.com>
Signed-off-by: Felipe Balbi <balbi@kernel.org>