tools/bootconfig: Fix the wrong format specifier

Use '%u' instead of '%d' for unsigned int.

Link: https://lore.kernel.org/all/20241105011048.201629-1-luoyifan@cmss.chinamobile.com/



Fixes: 97378001 ("tools/bootconfig: Suppress non-error messages")
Signed-off-by: Luo Yifan <luoyifan@cmss.chinamobile.com>
Signed-off-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>