Bluetooth: 6lowpan: Remove unnecessary chan_get() function

The chan_get() function just adds unnecessary indirection to calling
the chan_create() call. The only added value it gives is the chan->ops
assignment, but that can equally well be done in the calling code.

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>