Bluetooth: Fix confusion between parent and child channel for 6lowpan

The new_connection L2CAP channel callback creates a new channel based on
the provided parent channel. The 6lowpan code was confusingly naming the
child channel "pchan" and the parent channel "chan". This patch swaps
the names.

Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>