Fix platform counterpoll watermark test

[sanjair-git]

With the recent submodule sonic-utilities update #2807, 'test_counterpoll_watermark.py' test fails with the following error,

>               pytest_assert(expected in verified_output_dict[counterpoll], "{} is {}. expected to be {}"
                              .format(counterpoll, verified_output_dict[counterpoll], expected))
E               Failed: queue is enable. expected to be disable

• This PR helps in fixing the above issue by updating the WRED_ECN_QUEUE_STAT constant to the counterpoll constants file.

• Verified the fix and made sure test passes without any issues. Results below.