[qos_sai] portName referenced before assignment on t1-lag topo

[vcheketx]

Description

PR 6393 switched to using portName instead of iface for t1 topos.
Since portName is defined inside if loop, it results in variable referenced before assignment error if interfaces names do not contain '.'
Steps to reproduce the issue:

1. deploy t1-lag topo
2. run qos/test_qos_sai.py

Describe the results you received:

                if iface.startswith("Ethernet"):
                    if "." in iface:
                        portName, vlan_id = iface.split(".")
>                   portIndex = mgFacts["minigraph_ptf_indices"][portName]
E                   UnboundLocalError: local variable 'portName' referenced before assignment

Describe the results you expected:
Tests are passed.

Additional information you deem important:

**Output of `show version`:**

SONiC Software Version: SONiC.master.172539-dirty-20221110.175553
Distribution: Debian 11.5
Kernel: 5.10.0-12-2-amd64
Build commit: 7c746e67d
Build date: Thu Nov 10 18:01:32 UTC 2022
Built by: AzDevOps@sonic-build-workers-002D7H

Platform: x86_64-accton_as9516_32d-r0
HwSKU: newport
ASIC: barefoot
ASIC Count: 1

**Attach debug file `sudo generate_dump`:**

```
(paste your output here)
```

[azure-pipelines-wrapper]

Thanks for opening this issue!