[NETFILTER]: nfnetlink: remove unrequired check in nfnetlink_get_subsys

subsys_table is initialized to NULL, therefore just returns NULL in case
that it is not set.

Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>
Signed-off-by: Patrick McHardy <kaber@trash.net>
Signed-off-by: David S. Miller <davem@davemloft.net>