Since the USB OTG FIFO sizes are partly defined in 32-bit words and partly in bytes, the documentation of the of the USB OTG FIFO size definitions is extended by the respective unit.
Use `DWC2_USB_OTG_FS_TOTAL_FIFO_SIZE` instead of `USB_OTG_FS_TOTAL_FIFO_SIZE` since the latter is only defined in the vendor headers for STM32 MCUs. The STM32-specific problem that `USB_OTG_FS_TOTAL_FIFO_SIZE` is not defined in the vendor headers for all STM32 families has therefore been moved from the driver to the STM32-specific USB device header.
Make sure in `_usbdev_new_ep()` that `usbdev_ep_t::buf` is always aligned to 4
bytes. With this in mind, add intermediate casts to `uintptr_t` before casting
`usbdev_ep_t::buf` to `uint32_t *` to silence `-Wcast-align`, as we now manually
enforced correct alignment.
