mlxsw: spectrum_router: Add batch neighbour deletion

In commit 4a3c67a6 ("mlxsw: spectrum_router: Don't batch neighbour
deletion") I removed the support for batch deletion of neighbours on a
router interface (RIF) since at that time the firmware did not support
it for IPv6 neighbours.

This is now supported by the version enforced by the driver, so there is
no reason to delete neighbours one by one anymore.

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>