drivers: udc_stm32: only check for HS_SPEED if defined

Seems like not all stm32 devices define USBD_HS_SPEED in the HAL, only
check for USBD_HS_SPEED if defined. Fixes a build failure with the new
stack on F1 MCUs.

Signed-off-by: Fabio Baltieri <fabiobaltieri@google.com>