6LoWPAN: set proper netdev flags

This patch fixes settings for device initialization which makes possible to
use NDISC and TCP.

Signed-off-by: Alexander Smirnov <alex.bluesman.smirnov@gmail.com>
Acked-by: Dmitry Eremin-Solenikov <dbaryshkov@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>