Bluetooth: 6lowpan: Remove redundant BT_CONNECTED assignment

The L2CAP core code makes sure of setting the channel state to
BT_CONNECTED, so there's no need for the implementation code (6lowpan
in this case) to do it.

Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>
Acked-by: Jukka Rissanen <jukka.rissanen@linux.intel.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>