Bluetooth: Fix correct nesting for 6lowpan server channel

Server channels in BT_LISTEN state should use L2CAP_NESTING_PARENT. This
patch fixes the nesting value for the 6lowpan channel.

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