Draft: ucm2: Librem_5: Switch to headset mic on headphone plugin

This ensures that we switch to the headset microphone once a headphone is plugged in. We need to use the headphone jack at the moment since there's no separate mic detection yet.

To achieve this bump the headset priority to the same value as the headphone priority.

@martin.kepplinger was it deliberate that one has to switch this manually or due the fact that we don't detect mic atm?

To test: plug in a headset/headphone and see the input source switch in g-c-c's sound tab.