lib: sbi: sbi_ecall: Check the range of SBI error

We should also check if the return error code is greater than 0
(SBI_SUCCESS), as this is an invalid error.

Signed-off-by: Yu Chien Peter Lin <peterlin@andestech.com>
Reviewed-by: Xiang W <wxjstz@126.com>
Reviewed-by: Andrew Jones <ajones@ventanamicro.com>
Reviewed-by: Anup Patel <anup@brainfault.org>