[Mellanpx] Update SDK Sniffer default folder (sonic-net#3276)

- What I did
Change the target path for SDK Sniffer from "/var/log/mellanox/sniffer/" To: "/var/log/sdk_dbg"

- How I did it
Change the default for SDK_SNIFFER_TARGET_PATH

- How to verify it
Run SDK sniffer and make sure the sniffer output file kept in the new location

config/plugins/mlnx.py

@@ -42,7 +42,7 @@
 ENV_VARIABLE_SX_SNIFFER_TARGET = 'SX_SNIFFER_TARGET'
 
 # SDK sniffer file path and name
-SDK_SNIFFER_TARGET_PATH = '/var/log/mellanox/sniffer/'
+SDK_SNIFFER_TARGET_PATH = '/var/log/sdk_dbg/'
 SDK_SNIFFER_FILENAME_PREFIX = 'sx_sdk_sniffer_'
 SDK_SNIFFER_FILENAME_EXT = '.pcap'