igmp: stop looking up IPv4 multicast in the L2 table on igmp off

igmp_setup(), which `igmp off` also calls, enabled the lookup of IPv4
multicast in the L2 table. Groups learned while IGMP was on therefore
kept going only to the ports that had reported them, and with reports
no longer trapped nothing updated those entries until a reboot.

The lookup is now enabled in igmp_enable() and disabled in igmp_setup(),
so with IGMP off all IP multicast is flooded again.

rtl837x_igmp.c

@@ -107,8 +107,7 @@ void igmp_setup(void) __banked
 	REG_SET(RTL837X_IPV4_UNKN_MC_FLD_PMSK, mask);
 	REG_SET(RTL837X_IPV6_UNKN_MC_FLD_PMSK, mask);
 
-	// Enable lookup of IPv4 MC addresses in table
-	reg_bit_set(RTL837X_L2_CTRL, L2_CTRL_LUT_IPMC_HASH);
+	reg_bit_clear(RTL837X_L2_CTRL, L2_CTRL_LUT_IPMC_HASH);
 
 	/* Configure per-port IGMP operations when protocol messages are received
 	 * bits 0-9 enable MC protocol snooping
@@ -145,6 +144,8 @@ void igmp_enable(void) __banked
 {
 	print_string("igmp_enable called\n");
 	igmpEnabled = 1;
+	// Enable lookup of IPv4 MC addresses in table
+	reg_bit_set(RTL837X_L2_CTRL, L2_CTRL_LUT_IPMC_HASH);
 	// Configure trapping of unhandled IGMP protocol packets to CPU
 	REG_SET(RTL837X_IGMP_TRAP_CFG, IGMP_CPU_PORT | IGMP_TRAP_PRIORITY);