igmp: leave groups in 224.0.0.0/24 to flooding

The switch keeps flooding 224.0.0.0/24 with IGMP on, but a report for a
group in that block still made an entry for it, and the entry limited
the group to the ports that reported it. A host announcing mDNS on
224.0.0.251 thereby cut mDNS off from every other port. Such reports
are now ignored, as RFC 4541 asks.

rtl837x_igmp.h

@@ -10,6 +10,7 @@
 #define IGMP_V3_REPORT		0x22
 #define IGMP_V3_TO_INCLUDE	3
 #define IGMP_V3_TO_EXCLUDE	4
+#define IGMP_LOCAL_NET_0	224
 
 void igmp_setup(void) __banked;
 void igmp_enable(void) __banked;