igmp: only hand reports to the CPU while snooping is on

handle_rx() dispatched to igmp_packet_handler() on the destination
address alone, so an ordinary IGMPv3 report off the wire reached the
handler and could write a table entry whether or not anyone had asked
for snooping. The STP branch right above it is gated on stpEnabled;
this brings the IGMP branch in line.

Snooping state lived only in the per-port registers, and the receive
path cannot afford to read one per packet, so the flag shadows it:
igmp_enable() sets it, igmp_setup() clears it, and igmp_setup() runs
from both the boot path and "igmp off".

While here, igmp off becomes an explicit subcommand instead of the
fall-through, and an unrecognised igmp subcommand prints the usage
line rather than silently turning snooping off.

Six bytes of BANK1 and one of xdata, no internal RAM.

rtlplayground.c

@@ -121,6 +121,7 @@ __xdata uint16_t management_vlan;
 __xdata uint8_t tx_seq;
 
 __xdata uint8_t stpEnabled;
+__xdata uint8_t igmpEnabled;
 __xdata char hostname[24];	/* device hostname, default set at boot, see rtl837x_common.h */
 
 __code uint16_t bit_mask[16] = {
@@ -1152,7 +1153,7 @@ void handle_rx(void)
 				print_string("STP TX\n");
 				tcpip_output();
 			}
-		} else if (uip_buf[0] == 0x01 && uip_buf[1] == 0x00 && uip_buf[2] == 0x5e // IPv4-MC packet?
+		} else if (igmpEnabled && uip_buf[0] == 0x01 && uip_buf[1] == 0x00 && uip_buf[2] == 0x5e // IPv4-MC packet?
 			&& uip_buf[3] == 0x00 && uip_buf[4] == 0x00 && uip_buf[5] == 0x16) {
 			igmp_packet_handler();
 			if (uip_len) {