[NETFILTER]: nfctnetlink: assign helper to newly created conntrack

This fixes the bug which doesn't assign helper to newly created
conntrack via nf_conntrack_netlink.

Signed-off-by: Yasuyuki Kozakai <yasuyuki.kozakai@toshiba.co.jp>
Signed-off-by: Patrick McHardy <kaber@trash.net>
Signed-off-by: David S. Miller <davem@davemloft.net>