target/riscv: Propagate error from PMU setup

More closely follow the QEMU style by returning an Error and propagating
it there is an error relating to the PMU setup.

Further simplify the function by removing the num_counters parameter as
this is available from the passed in cpu pointer.

Signed-off-by: Rob Bradford <rbradford@rivosinc.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Reviewed-by: LIU Zhiwei <zhiwei_liu@linux.alibaba.com>
Reviewed-by: Atish Patra <atishp@rivosinc.com>
Message-ID: <20231031154000.18134-2-rbradford@rivosinc.com>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>