net: if: Setup DAD timer regardless of DAD query result

In rare occasions when sending DAD NS packet fails, we should still
setup the DAD timer, unless we implement some kind of more advanced
retry mechanism. If we don't do that, the IPv6 address added to the
interface will never be usable in such cases.

Signed-off-by: Robert Lubos <robert.lubos@nordicsemi.no>
(cherry picked from commit 008a7ca2)