Bluetooth: Functions for handling ERTM control fields

These functions encode or decode ERTM control fields (extended or
enhanced) to or from the new l2cap_ctrl structure.

Signed-off-by: Mat Martineau <mathewm@codeaurora.org>
Acked-by: Andrei Emeltchenko <andrei.emeltchenko@intel.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>