lib: fix fdt_parse_plicsw_node()

cpu_offset and cpu_intc_offset must be int to detect failed invocations of
fdt_node_offset_by_phandle() or fdt_parent_offset().

After determining cpu_offset we have to check this value and not
cpu_intc_offset.

Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>
Reviewed-by: Xiang W <wxjstz@126.com>
Reviewed-by: Yu Chien Peter Lin <peterlin@andestech.com>
Reviewed-by: Bin Meng <bmeng@tinylab.org>