lib: fix irqchip_plic_update_hartid_table

After determining cpu_offset we have to check this value.

Addresses-Coverity-ID: 1529706 ("Logically dead code")
Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>
Reviewed-by: Xiang W <wxjstz@126.com>
Reviewed-by: Atish Patra <atishp@rivosinc.com>
Reviewed-by: Anup Patel <anup@brainfault.org>