drivers: usb: device: simplify code of maximum-speed

Add defines to avoid magic values in the dtsi.
Replace usb_dc_stm32_get_maximum_speed() with device tree property.
- Use DT_INST_STRING_UPPER_TOKEN(0, maximum_speed) to set the USB speed.
- Remove PCD_SPEED_FULL initialization.

Signed-off-by: Khaoula Bidani <khaoula.bidani-ext@st.com>