samples: net: lwm2m_client: Bump IPv6 mcast address count

The multicast IPv6 address count was too low to create all
solicited-node multicast address, resulting in an error being
printed on boot:

  Cannot join solicit node address ff02::1:ff00:1 for 1 (-12)

Signed-off-by: Robert Lubos <robert.lubos@nordicsemi.no>