Bluetooth: Add CONFIG_BT_LE_L2CAP_ECRED

This adds CONFIG_BT_LE_L2CAP_ECRED which can be used to enable L2CAP
Enhanced Credit Flow Control Mode by default, previously it was only
possible to set it via module parameter (e.g. bluetooth.enable_ecred=1).

Since L2CAP ECRED mode is required by the likes of EATT which is
recommended for LE Audio this enables it by default.

Signed-off-by: Luiz Augusto von Dentz <luiz.von.dentz@intel.com>
Tested-By: Tedd Ho-Jeong An <tedd.an@intel.com>