firmware: Remove the sanity checks in fw_save_info()

The sanity checks on the magic and version was already done in
fw_boot_hart(), which happens before fw_save_info() is called.

Signed-off-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Xiang W <wxjstz@126.com>
Reviewed-by: Anup Patel <anup.patel@wdc.com>