Bluetooth: L2CAP: Add initial code for Enhanced Credit Based Mode

This adds the initial code for Enhanced Credit Based Mode which
introduces a new socket mode called L2CAP_MODE_EXT_FLOWCTL, which for
the most part work the same as L2CAP_MODE_LE_FLOWCTL but uses different
PDUs to setup the connections and also works over BR/EDR.

Signed-off-by: Luiz Augusto von Dentz <luiz.von.dentz@intel.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>