net: ipv6: Add missing const to ipv6_chk_addr

net: ipv6: Add missing const to ipv6_chk_addr

The net_device used by ipv6_chk_addr is const in some users so add a
const here, fixing a warning following mainline commit 6bc19fb82d4c05
(Merge git://git.kernel.org/pub/scm/linux/kernel/git/davem/net) from
David S. Miller.

Change-Id: Iebfae34fc9f10f958c41aad6ac4008d61668a870
Signed-off-by: Mark Brown <broonie@linaro.org>
Signed-off-by: Sherman Yin <syin@broadcom.com>
[Folded in additional const fix to the dummy function from Sherman Yin]
Signed-off-by: John Stultz <john.stultz@linaro.org>