hw/nvram/fw_cfg: Remove support for I/O port fw_cfg without DMA

Currently fw_cfg_init_io_dma() allows the caller to pass a NULL dma_as argument, which causes it to create a fw_cfg without the DMA port or DMA support. None of the callers use this capability: they all pass &address_space_memory. We don't really want to leave the door open for some future x86 machine type which doesn't support DMA for the fw_cfg device, so remove this, and instead make the function assert that it has a non-NULL dma_as argument, like fw_cfg_init_mem_dma(). Signed-off-by: Peter Maydell <peter.maydell@linaro.org> Reviewed-by: Richard Henderson <richard.henderson@linaro.org> Reviewed-by: Philippe Mathieu-Daudé <philmd@oss.qualcomm.com> Reviewed-by: Philippe Mathieu-Daudé <philmd@mailo.com> Message-id: 20260529174639.451353-5-peter.maydell@linaro.org Reviewed-by: Michael S. Tsirkin <mst@redhat.com>