The enum value of cipher of ExpressRouteLinkMacSecConfig that service returned didn't match spec's saying

According to [this file](https://github.com/Azure/azure-rest-api-specs/blob/master/specification/network/resource-manager/Microsoft.Network/stable/2019-08-01/expressRoutePort.json), the cipher value service returned should be in [ "gcm-aes-128", "gcm-aes-256" ], but instead: [ "GcmAes128", "GcmAes256" ].

```
"ExpressRouteLinkMacSecConfig": {
"description": "ExpressRouteLink Mac Security Configuration.",
"title": "Definition of ExpressRouteLink Mac Security configuration.",
"properties": {
"cknSecretIdentifier": {
"type": "string",
"description": "Keyvault Secret Identifier URL containing Mac security CKN key."
},
"cakSecretIdentifier": {
"type": "string",
"description": "Keyvault Secret Identifier URL containing Mac security CAK key."
},
"cipher": {
"type": "string",
"description": "Mac security cipher.",
"enum": [
"gcm-aes-128",
"gcm-aes-256"
],
"x-ms-enum": {
"name": "ExpressRouteLinkMacSecCipher",
"modelAsString": true
}
}
}
}
```

The problem also occurs when I tried to update the Port links with cipher value, either "gcm-aes-128", "gcm-aes-256" is not acceptable by the service, but "gcmaes128", "gcmaes256" is recognized only.

Please correct them ASAP, cause it's related to this [issue](https://github.com/Azure/azure-cli/issues/10747).

Start with specification/network/resource-manager/Microsoft.Network/stable/2019-08-01/expressRoutePort.json and compare its ExpressRouteLinkMacSecConfig cipher enum with the service behavior described here. Review the linked Azure CLI issue 10747 for related context. Done means the specification and accepted service values are aligned and the discrepancy is resolved.

Written by the indexing model from the issue text.