mlxsw: spectrum_router: Prepare nexthop group's hash table for IPv6

This patch does preparation before introducing IPv6 nexthop group
consolidation. Currently the nexthop group hash table is used only by
IPv4 and uses fixed key size. In order to support the IPv6's variable
length key the current table is changed.

Signed-off-by: Arkadi Sharshevsky <arkadis@mellanox.com>
Reviewed-by: Ido Schimmel <idosch@mellanox.com>
Signed-off-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>