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: I6efe76666f75fb4f65621796466d9347cea25d01
diff --git a/.clang-tidy b/.clang-tidy
index dd69f53..43d168b 100644
--- a/.clang-tidy
+++ b/.clang-tidy
@@ -254,6 +254,7 @@
 performance-type-promotion-in-math-fn,
 performance-unnecessary-copy-initialization,
 readability-const-return-type,
+readability-convert-member-functions-to-static,
 readability-delete-null-pointer,
 readability-deleted-default,
 readability-function-size,