if your system is read-only then you should make a build with debug option enabled:
inside your buildroot create this path: files/etc/modules.d/ and put file named ath9k there which should contain "ath9k debug=0xffffffff" (more info: https://openwrt.org/docs/guide-developer/build-system/use-buildsystem#custom_files) rebuild and try boot again capturing bootlog
