linux-postmarketos-allwinner: downgrade to 6.7.9_git20240306 (MR 5054)

Kernel 6.8.4 has multiple issues on the pine64-pinephone:

- Incoming calls don't work reliably when the phone is suspended. ( #2734 )

- Audio routing has changed such that it's seemingly not possible to use
  headphones with the builtin mic. ( #2738 )

Kernel 6.7.9 does not have these problems. This commit reverts to that for now
until they're figured out on the new kernel.