RISC-V: hwprobe: There can only be one first

Only capture the first cpu_id in order for the comparison
below to be of any use.

Fixes: ea3de9ce ("RISC-V: Add a syscall for HW probing")
Signed-off-by: Andrew Jones <ajones@ventanamicro.com>
Reviewed-by: Conor Dooley <conor.dooley@microchip.com>
Reviewed-by: Evan Green <evan@rivosinc.com>
Link: https://lore.kernel.org/r/20230426141333.10063-2-ajones@ventanamicro.com


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