mlxsw: spectrum_router: Fix netevent notifier registration

Currently the notifier is registered for every asic instance, however the
same block. Fix this by moving the registration to module init.

Fixes: c723c735 ("mlxsw: spectrum_router: Periodically update the kernel's neigh table")
Signed-off-by: Jiri Pirko <jiri@mellanox.com>
Reviewed-by: Ido Schimmel <idosch@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>