rhashtable_lookup_fast() returns NULL when failed instead of error
pointer.

Fixes: 396b80cb ("net: marvell: prestera: Add neighbour cache accounting")
Fixes: 0a23ae23 ("net: marvell: prestera: Add router nexthops ABI")
Signed-off-by: NShang XiaoJing <shangxiaojing@huawei.com>
Link: https://lore.kernel.org/r/20221125012751.23249-1-shangxiaojing@huawei.comSigned-off-by: NPaolo Abeni <pabeni@redhat.com>