lib: sbi: Fix error codes returned by HSM start() and stop() functions

The sbi_hsm_hart_start() and sbi_hsm_hart_stop() functions should
only return error codes as defined by the SBI specification.

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