lib: utils: check correct value in fdt_node_offset_by_compatible

After calling fdt_node_offset_by_compatible() we must check its return
value and not an unrelated value.

Addresses-Coverity-ID: 1584993 Logically dead code
Fixes: 67ce5a76

 ("platform: generic: Add support for specify coldboot harts in DT")
Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>
Reviewed-by: Anup Patel <anup@brainfault.org>