mlxsw: spectrum_router: Use correct config option

I made an embarrassing mistake and used 'IPV6' instead of 'CONFIG_IPV6'
around the function that updates the kernel about IPv6 neighbours
activity. This can be a problem if the kernel has more neighbours than a
certain threshold and it starts deleting those that are supposedly
inactive.

Fixes: b5f3e0d4

 ("mlxsw: spectrum_router: Fix build when IPv6 isn't enabled")
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>