STM32 OTG HS: A little research reveals that only the F2 RCC initialization...

STM32 OTG HS:  A little research reveals that only the F2 RCC initialization set the OTGHSULPIEN bit and Photon is the only F2 board configuration that uses OTG .  Therefore, we can simplify the conditional logic of the last PR.  Negative logic was used (#ifndef BOARD_DISABLE_USBOTG_HSULPI) to prevent bad settings in other configurations.  But give these facts, the preferred positive logic now makes more sense (#ifdef BOARD_ENABLE_USBOTG_HSULPI).