create sniffer folder if not exist (sonic-net#1659)

- What I did
following the change in PR sonic-net#7830, the folder needs to be created when the first time enables the sniffer function.

- How I did it
check whether the path to store the sniffer file is existing or not, if not, create it.

- How to verify it
run command "config platform mlnx sniffer sdk enable", can see sniffer function can be enabled successfully.

config/plugins/mlnx.py

@@ -43,6 +43,8 @@
 
 # generate sniffer target file name include a time stamp.
 def sniffer_filename_generate(path, filename_prefix, filename_ext):
+    if not os.path.exists(path):
+        os.makedirs(path)
     time_stamp = time.strftime("%Y%m%d%H%M%S")
     filename = path + filename_prefix + time_stamp + filename_ext
     return filename