Bluetooth: Introduce L2CAP channel flag for defer setup

The L2CAP core should not look into the socket flags to figure out the
setting of defer setup. So introduce a L2CAP channel flag that mirrors
the socket flag.

Since the defer setup option is only set in one place this becomes a
really easy thing to do.

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