[202012] [Mellanox] Fix select timeout in sfp event (#13347)

- Why I did it
Backport #9795
Python select.select accept a optional timeout value in seconds, however, the value passes to it is a value in millisecond.

- How I did it
Transfer the value to millisecond.

- How to verify it
Manual test

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

@@ -217,7 +217,7 @@ class sfp_event:
         found = 0
 
         try:
-            read, _, _ = select.select([self.rx_fd_p.fd], [], [], timeout)
+            read, _, _ = select.select([self.rx_fd_p.fd], [], [], float(timeout) / 1000)
         except select.error as err:
             rc, msg = err
             if rc == errno.EAGAIN or rc == errno.EINTR: