x86: bios: Synchronize stack between real and protected mode

PCI option rom may use different SS during its execution, so it is not
safe to assume esp pointed to the same location in the protected mode.

Signed-off-by: Jian Luo <jian.luo4@boschrexroth.de>
Signed-off-by: Bin Meng <bmeng.cn@gmail.com>
Acked-by: Simon Glass <sjg@chromium.org>