lib: sbi_hsm: Don't use sbi_platform_hart_disabled() API

Checking return value of sbi_hartid_to_scratch() is sufficient
so no need to explicitly check for disabled HART using the
sbi_platform_hart_disabled() API.

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>