riscv: fix riscv_of_processor_hartid() comment

The riscv_of_processor_hartid() helper returns -ENODEV when the
specified node isn't an enabled and valid RISC-V hart node.

Also drop the unnecessary parenthesis around errno defines.

Signed-off-by: Johan Hovold <johan@kernel.org>
Reviewed-by: Paul Walmsley <paul.walmsley@sifive.com>
Signed-off-by: Palmer Dabbelt <palmer@sifive.com>