cpu/stm32/periph/usbdev_fs: always use 16-bit addresses for PMA
cpu/stm32/periph/usbdev_fs: always use 16-bit addresses for PMA

The addressing of the Packet buffer Memory Area (PMA) is done locally in the USB IP core in half-words with 16-bit. The `_ep_in_buf` and `_ep_out_buf` arrays which hold these USB IP local addresses in the PMA for initialized EPs therefore always use `uint16_t`.
jasonatran/RIOT,RIOT-OS/RIOT,kaspar030/RIOT,ant9000/RIOT,RIOT-OS/RIOT,jasonatran/RIOT,kaspar030/RIOT,RIOT-OS/RIOT,ant9000/RIOT,RIOT-OS/RIOT,kaspar030/RIOT,ant9000/RIOT,ant9000/RIOT,RIOT-OS/RIOT,ant9000/RIOT,kaspar030/RIOT,kaspar030/RIOT,jasonatran/RIOT,jasonatran/RIOT,jasonatran/RIOT