[Mellanox] Redirect ethtool stderr to subprocess for better error log (#12038)

- Why I did it
ethtool print error logs when EEPROM of a SFP is not available. It prints error like this:

INFO pmon#/supervisord: xcvrd Cannot get module EEPROM information: Input/output error
INFO pmon#/supervisord: xcvrd Cannot get Module EEPROM data: Invalid argument
However, this log does not contain the relevant SFP index which is hard for developer/qa to find the exactly SFP.

- How I did it
Redirect ethtool stderr to subprocess and log it better

- How to verify it
Manual test

platform/mellanox/mlnx-platform-api/sonic_platform/sfp.py

@@ -359,7 +359,8 @@ class SFP(NvidiaSFPCommon):
         try:
             output = subprocess.check_output(ethtool_cmd,
                                              shell=True,
-                                             universal_newlines=True)
+                                             universal_newlines=True,
+                                             stderr=subprocess.PIPE)
             output_lines = output.splitlines()
             first_line_raw = output_lines[0]
             if "Offset" in first_line_raw:
@@ -367,6 +368,7 @@ class SFP(NvidiaSFPCommon):
                     line_split = line.split()
                     eeprom_raw = eeprom_raw + line_split[1:]
         except subprocess.CalledProcessError as e:
+            logger.log_notice("Failed to get EEPROM data for sfp {}: {}".format(self.index, e.stderr))
             return None
 
         eeprom_raw = list(map(lambda h: int(h, base=16), eeprom_raw))