usb: device_next: Fix Mass Storage on STM32

Mass Storage enumeration failed because udc_stm32_ep_mem_config() was only
increasing priv->occupied_mem on endpoint enable, and not decreasing it on
endpoint disable. Fix the issue by decreasing priv->occupied_mem
on endpoint disable.

Signed-off-by: Arne Bohnsack <arne.bohnsack@draeger.com>