[IPV6] NDISC: Fix setting base_reachable_time_ms variable.

This bug was introduced by the commit
d12af679 (sysctl: fix neighbour table
sysctls).

Signed-off-by: YOSHIFUJI Hideaki <yoshfuji@linux-ipv6.org>
Signed-off-by: David S. Miller <davem@davemloft.net>