x86: fsp: Show FSP-S or FSP-M address in fsp_get_header()

At present this function only supports FSP-M but it is also used to read
FSP-S, in which case FSP-M may be zero. Add support for showing whichever
address is present in the FSP binary.

Also change the debug() statements to log_debug() while here.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>