Align description with https://github.com/ietf-ccamp-wg/draft-ietf-ccamp-optical-impairment-topology-yang/issues/43

[italobusi]

The description of the operational-mode is not aligned with https://github.com/ietf-ccamp-wg/draft-ietf-ccamp-optical-impairment-topology-yang/issues/43:

https://github.com/ietf-ccamp-wg/ietf-ccamp-layer0-types-ext/blob/46433aa5279ccd559c17b7c6bea6c44547a99282/ietf-layer0-types-ext.yang#L162-L180

[italobusi]

Moreover:

• the reference to G.698.2 seems not applicable to the operational-mode
• it is better to reference G.698.2 for the format for the standard-mode string to avoid inconsistencies

It is proposed to keep the interoperability description in the I-D text and to focus the YANG description on how to format the string:

  typedef operational-mode {
    type string;
    description
      "Identifies an organization (e.g., vendor) specific mode.

       The format of this string has to be defined by the
       organization which is responsible for defining the
       corresponding optical interface specification.";
  }

  typedef standard-mode {
    type string;
    description
      "Identifies an ITU-T G.698.2 standard application code.

       It MUST be a string with a format that follows the nomenclature defined in section 5.3 of ITU-T G.698.2.";
    reference "ITU-T G.698.2 (11/2018)";
  }