samples: ipsp: increase the maximum multicast addr count

This sample is trying to register four multicast addesses:

uart:~$ net iface
...
IPv6 multicast addresses (max 8):
        ff02::1
        ff02::1:ff01:c41e
        ff02::1:ff00:1
        ff02::1

Increasing CONFIG_NET_IF_MCAST_IPV6_ADDR_COUNT to 4 to make neighbor
discovery work correctly.

Signed-off-by: Fabio Baltieri <fabio.baltieri@gmail.com>