fw_cfg: fix sparse warnings around FW_CFG_FILE_DIR read

Use struct fw_cfg_files to read the directory size, fixing the sparse
warnings:

drivers/firmware/qemu_fw_cfg.c:485:17: warning: cast to restricted __be32

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>