lib: Don't use sbi_platform_hart_count() API

We don't need to use sbi_platform_hart_count() in sbi_init and
sbi_scratch because checking sbi_platform_hart_disabled() or
return value of sbi_hartid_to_scratch() is sufficient.

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>