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: 47aeea0d ("net: lan966x: Implement the callback SWITCHDEV_ATTR_ID_BRIDGE_MC_DISABLED")
Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Horatiu Vultur <horatiu.vultur@microchip.com>
Signed-off-by: David S. Miller <davem@davemloft.net>