Fix issue: test case should be marked as failure if cannot get PSU fan speed (#3015)

Description of PR
Summary:
Test case should be marked as failure if cannot get PSU fan speed

Approach
What is the motivation for this PR?
Test case should be marked as failure if cannot get PSU fan speed because it usually means that there is an issue with the sysfs.

How did you do it?
If PSU speed sysfs does not exist, assert false.

How did you verify/test it?
Manually run the test case

Any platform specific information?
Mellanox

Author: Junchao-Mellanox

tests/platform_tests/mellanox/test_thermal_control.py

@@ -131,8 +131,7 @@ def get_psu_speed(dut, index):
     index = index + 1
     psu_speed_path = PSU_SPEED_PATH.format(index)
     file_stat = dut.stat(path=psu_speed_path)
-    if not file_stat["stat"]["exists"]:
-        return None
+    assert file_stat["stat"]["exists"], 'Failed to get PSU speed file due to {} does not exist'.format(psu_speed_path)
 
     cmd_output = dut.command('cat {}'.format(psu_speed_path))
     try: