net: if: No need to always join solicit node mcast group

For example for Bluetooth IPSP, it is not needed to join solicited
node multicast group address.

From https://tools.ietf.org/html/rfc7668#section-3.2.2

 :

"""
There is no need for 6LN to join the solicited-node multicast address,
since 6LBR will know device addresses and hence link-local addresses
of all connected 6LNs.
"""

Signed-off-by: Jukka Rissanen <jukka.rissanen@linux.intel.com>