Bluetooth: Add LE L2CAP flow control mode

The LE connection oriented channels have their own mode with its own
data transfer rules. In order to implement this properly we need to
distinguish L2CAP channels operating in this mode from other modes.

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