lib: Initialize out value in SBI calls

As per the SBI specification, the return value in sbiret is undefined
if not explicitly described in the function. However, supervisor may
check this value by mistake and get a garbage value.

Initialize it to zero to avoid nasty supervisor bugs.

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