fw_cfg: move setting of FW_CFG_VERSION_DMA bit to fw_cfg_init1()

The setting of the FW_CFG_VERSION_DMA bit is the same across both the
TYPE_FW_CFG_MEM and TYPE_FW_CFG_IO devices, so unify the logic in
fw_cfg_init1().

Signed-off-by: Mark Cave-Ayland <mark.cave-ayland@ilande.co.uk>
Reviewed-by: Laszlo Ersek <lersek@redhat.com>
Reviewed-by: Eduardo Habkost <ehabkost@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Tested-by: Gabriel Somlo <somlo@cmu.edu>