Update checkpatch to run only when kernel files are touched

Updating the checkpatch github action so that it only runs when we
modify kernel related files (ex: Anything in 3.17/4.5 sub directories).

This change keeps the tests run during a PR cleaner by only running
when it is most relevant for code styling/commit messages that are
relevant to keeping style parity with upstream, while ignoring
irrelevant changes to files that aren't upstream (such as this one).

.github/workflows/checkpatch.yml

@@ -1,4 +1,8 @@
-on: [ pull_request ]
+on:
+  pull_request:
+    paths:
+    - '3.17/**'
+    - '4.5/**'
 
 jobs:
   checkpatch: