mlxsw: spectrum_router: Prepare for large adjacency groups

The device has certain restrictions regarding the size of an adjacency
group.

Have the router determine the size of the adjacency group according to
available KVDL allocation sizes and these restrictions.

This was not needed until now since only allocations of up 32 entries
were supported and these are all valid sizes for an adjacency group.

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