net/net_if: When IPv6 DAD is disabled, added address gets final state

There is no transition from tentative state etc... It should directly go
to preferred.

This is fixing ND processing when DAD is disabled: source address was
never set, as tentative state is not a valid in
is_proper_ipv6_address().

Change-Id: I6f0a0fdd99dd13d28849f9749f89ec21fdd370d8
Signed-off-by: Tomasz Bursztyka <tomasz.bursztyka@linux.intel.com>