drivers: usb: usb_dc_stm32: Rename defines to remove DT_ prefix

We want to limit DT_ prefix to macros from devicetree.h and generation.
So rename DT_USB* to just USB*.

Also fixup how USB_MAXIMUM_SPEED was defined.  We should only define it
if the property exists.

Signed-off-by: Kumar Gala <kumar.gala@linaro.org>