MIPS: Fix the wrong format specifier

Make a minor change to eliminate a static checker warning. The type
of cpu is unsigned int, so the correct format specifier should be
%u instead of %d.

Signed-off-by: liujing <liujing@cmss.chinamobile.com>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>