RISC-V: Fix error code returned by riscv_hartid_to_cpuid()

We should return a negative error code upon failure in
riscv_hartid_to_cpuid() instead of NR_CPUS. This is also
aligned with all uses of riscv_hartid_to_cpuid() which
expect negative error code upon failure.

Fixes: 6825c7a8 ("RISC-V: Add logical CPU indexing for RISC-V")
Fixes: f99fb607

 ("RISC-V: Use Linux logical CPU number instead of hartid")
Signed-off-by: Anup Patel <anup.patel@wdc.com>
Signed-off-by: Palmer Dabbelt <palmerdabbelt@google.com>