clang-tidy: Enable readability-convert-member-functions-to-static

This check finds non-static member functions that can be made
static because the functions don’t use this.
This check also triggers readability-static-accessed-through
-instance check as we are trying to access a static member
function through an instance.

Signed-off-by: George Liu <liuxiwei@ieisystem.com>
Change-Id: I6c06c61e8f199247e74aca3443741683e3853274
diff --git a/.clang-tidy b/.clang-tidy
index b3cb96d..ad408a4 100644
--- a/.clang-tidy
+++ b/.clang-tidy
@@ -230,6 +230,7 @@
 readability-braces-around-statements,
 readability-const-return-type,
 readability-container-size-empty,
+readability-convert-member-functions-to-static,
 readability-delete-null-pointer,
 readability-deleted-default,
 readability-function-size,