net: lan966x: Fix when CONFIG_IPV6 is not set

When CONFIG_IPV6 is not set, then the linking of the lan966x driver
fails with the following error:

drivers/net/ethernet/microchip/lan966x/lan966x_main.c:444: undefined
reference to `ipv6_mc_check_mld'

The fix consists in adding a check also for IS_ENABLED(CONFIG_IPV6)

Fixes: 47aeea0d57e80c ("net: lan966x: Implement the callback SWITCHDEV_ATTR_ID_BRIDGE_MC_DISABLED")
Reported-by: kernel test robot <[e-mail]>
Signed-off-by: Horatiu Vultur <[e-mail]>
Signed-off-by: David S. Miller <[e-mail]>

drivers/net/ethernet/microchip/lan966x/lan966x_main.c

···
 	    ip_hdr(skb)->protocol == IPPROTO_IGMP)
 		return false;
 
-	if (skb->protocol == htons(ETH_P_IPV6) &&
+	if (IS_ENABLED(CONFIG_IPV6) &&
+	    skb->protocol == htons(ETH_P_IPV6) &&
 	    ipv6_addr_is_multicast(&ipv6_hdr(skb)->daddr) &&
 	    !ipv6_mc_check_mld(skb))
 		return false;