[202205] [Mellanox] Fix typo in platform API (#12139)

- Why I did it
Fix a typo in chassis platform API which causes the following error

>>> import sonic_platform as P
>>> c = P.platform.Platform().get_chassis()
>>> sl = c.get_all_sfps()
>>> sl[0].get_lpmode()
Sep 28 07:48:33 INFO    LOG: Initializing SX log with STDOUT as output file.
False
>>> del c
Exception ignored in: <function Chassis.__del__ at 0x7f1d166ef8b0>
Traceback (most recent call last):
  File "/usr/local/lib/python3.9/dist-packages/sonic_platform/chassis.py", line 126, in __del__
    self.sfp_module.deinitialize_sdk_handle(sfp_module.SFP.shared_sdk_handle)
NameError: name 'sfp_module' is not defined

- How I did it
Use self while using the SDK handle

- How to verify it
Manual test

Signed-off-by: Stephen Sun <stephens@nvidia.com>

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

@@ -123,7 +123,7 @@ class Chassis(ChassisBase):
 
         if self._sfp_list:
             if self.sfp_module.SFP.shared_sdk_handle:
-                self.sfp_module.deinitialize_sdk_handle(sfp_module.SFP.shared_sdk_handle)
+                self.sfp_module.deinitialize_sdk_handle(self.sfp_module.SFP.shared_sdk_handle)
 
     @property
     def RJ45_port_list(self):