lib: Handle failure of sbi_hartid_to_scratch() API

The sbi_hartid_to_scratch() API can fail for non-existent HARTs so
all uses of sbi_hartid_to_scratch() API should check return value.

Signed-off-by: Anup Patel <anup.patel@wdc.com>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Atish Patra <atish.patra@wdc.com>