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>

drivers/firmware/qemu_fw_cfg.c

@@ -496,19 +496,20 @@ static int fw_cfg_register_file(const struct fw_cfg_file *f)
 static int fw_cfg_register_dir_entries(void)
 {
 	int ret = 0;
+	__be32 files_count;
 	u32 count, i;
 	struct fw_cfg_file *dir;
 	size_t dir_size;
 
-	fw_cfg_read_blob(FW_CFG_FILE_DIR, &count, 0, sizeof(count));
-	count = be32_to_cpu(count);
+	fw_cfg_read_blob(FW_CFG_FILE_DIR, &files_count, 0, sizeof(files_count));
+	count = be32_to_cpu(files_count);
 	dir_size = count * sizeof(struct fw_cfg_file);
 
 	dir = kmalloc(dir_size, GFP_KERNEL);
 	if (!dir)
 		return -ENOMEM;
 
-	fw_cfg_read_blob(FW_CFG_FILE_DIR, dir, sizeof(count), dir_size);
+	fw_cfg_read_blob(FW_CFG_FILE_DIR, dir, sizeof(files_count), dir_size);
 
 	for (i = 0; i < count; i++) {
 		ret = fw_cfg_register_file(&dir[i]);