ieee802154: 6lowpan: change if lowpan dev is running

We don't need to check if the wpan interface is running because the
lowpan_rcv is the packet layer receive handler for the wpan interface.

Instead doing a check if wpan interface is running we should check if
the lowpan interface is running before starting 6lowpan adaptation layer.

Reviewed-by: Stefan Schmidt <stefan@osg.samsung.com>
Signed-off-by: Alexander Aring <alex.aring@gmail.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>