riscv: Use '%u' to format the output of 'cpu'

'cpu' is an unsigned integer, so its conversion specifier should
be %u, not %d.

Suggested-by: Wentao Guan <guanwentao@uniontech.com>
Suggested-by: Maciej W. Rozycki <macro@orcam.me.uk>
Link: https://lore.kernel.org/all/alpine.DEB.2.21.2409122309090.40372@angie.orcam.me.uk/


Signed-off-by: WangYuli <wangyuli@uniontech.com>
Reviewed-by: Charlie Jenkins <charlie@rivosinc.com>
Tested-by: Charlie Jenkins <charlie@rivosinc.com>
Fixes: f1e58583 ("RISC-V: Support cpu hotplug")
Cc: stable@vger.kernel.org
Link: https://lore.kernel.org/r/4C127DEECDA287C8+20241017032010.96772-1-wangyuli@uniontech.com


Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>