Fix decode error when parsing EEPROM fields (sonic-net#199)

#### Description
Following error is seen while parsing EEPROM fields for few transceivers.
'utf-8' codec can't decode byte 0xff in position 6: invalid start byte

#### Motivation and Context
EEPROM fields will not be displayed  if unicode characters are not properly.
E.g. Vendor Date: 20'u-tf--8'

sonic_platform_base/sonic_sfp/sffbase.py

@@ -35,7 +35,7 @@ def convert_hex_to_string(self, arr, start, end):
             ret_str = ''
             for n in range(start, end):
                 ret_str += arr[n]
-            return binascii.unhexlify(ret_str).decode("utf-8").strip()
+            return binascii.unhexlify(ret_str).decode("utf-8", "ignore").strip()
         except Exception as err:
             return str(err)