[qos_sai] Fix qos_sai portName referenced before assignment

[vcheketx]

Description of PR

Summary:
Fixes #6864

Type of change

• Bug fix
• Testbed and Framework(new/improvement)
• Test case(new/improvement)

Back port request

• 201911
• 202012
• 202205

Approach

What is the motivation for this PR?

On t1-lag topo, qos_sai is failed due to portName referenced before assignment.

How did you do it?

Added definition outside of if loop.

How did you verify/test it?

Run qos/test_qos_sai.py:
qos/test_qos_sai.py::TestQosSai::testParameter[cab18-4-dut-None] PASSED

Any platform specific information?

Supported testbed topology if it's a new test case?

Documentation

[azure-pipelines]

The pre-commit check detected issues in the files touched by this pull request.
The detected issues may be old or new. For new issues, please try to fix them.

For old issues, it is not mandatory to fix them because they were not caused by this change. It is unfair to blame
author of this pull request. But if you can take extra effort to fix the old issues as well, that would be great!

Detailed pre-commit check results:
trim trailing whitespace.................................................Failed
- hook id: trailing-whitespace
- exit code: 1
- files were modified by this hook

Fixing tests/qos/qos_sai_base.py

fix end of files.........................................................Passed
check yaml...........................................(no files to check)Skipped
check for added large files..............................................Passed
check python ast.........................................................Passed
flake8...................................................................Failed
- hook id: flake8
- exit code: 1

tests/qos/qos_sai_base.py:7:1: F401 'tests.common.fixtures.ptfhost_utils.ptf_portmap_file' imported but unused
tests/qos/qos_sai_base.py:10:1: F401 'tests.common.dualtor.dual_tor_utils.upper_tor_host' imported but unused
tests/qos/qos_sai_base.py:10:1: F401 'tests.common.dualtor.dual_tor_utils.lower_tor_host' imported but unused
tests/qos/qos_sai_base.py:10:1: F401 'tests.common.dualtor.dual_tor_utils.dualtor_ports' imported but unused
tests/qos/qos_sai_base.py:10:63: E231 missing whitespace after ','
...
[truncated extra lines, please run pre-commit locally to view full check results]

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