Bluetooth: 6lowpan: Remove unnecessary chan_open() function

All the chan_open() function now does is to call chan_create() so it
doesn't really add any value.

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>