net: marvell: prestera: Fix return value check in prestera_kern_fib_cache_find()

rhashtable_lookup_fast() returns NULL pointer not ERR_PTR(), so
it can return fib_node directly in prestera_kern_fib_cache_find().

Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: Yang Yingliang <yangyingliang@huawei.com>
Link: https://lore.kernel.org/r/20220223084954.1771075-2-yangyingliang@huawei.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>