[dfs] Update DFS_FD_MAX default size to 16.

components/dfs/Kconfig

@@ -24,8 +24,7 @@ if RT_USING_DFS
 
     config DFS_FD_MAX
         int "The maximal number of opened files"
-        default 16 if RT_USING_DFS_NFS
-        default 4
+        default 16
 
     config RT_USING_DFS_ELMFAT
         bool "Enable elm-chan fatfs"