lib: Fix return type of sbi_hsm_hart_started()

The return type of sbi_hsm_hart_started() should be bool.

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