[IPV6] NDISC: Fix thinko to control Router Preference support.

Bug reported by Haruhito Watanabe <haruhito@sfc.keio.ac.jp>.
Signed-off-by: NYOSHIFUJI Hideaki <yoshfuji@linux-ipv6.org>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/ipv6/ndisc.c

@@ -1062,7 +1062,7 @@ static void ndisc_router_discovery(struct sk_buff *skb)
 	pref = ra_msg->icmph.icmp6_router_pref;
 	/* 10b is handled as if it were 00b (medium) */
 	if (pref == ICMPV6_ROUTER_PREF_INVALID ||
-	    in6_dev->cnf.accept_ra_rtr_pref)
+	    !in6_dev->cnf.accept_ra_rtr_pref)
 		pref = ICMPV6_ROUTER_PREF_MEDIUM;
 #endif