[Keyboard] Fix default keymaps for OLKB boards to play Startup Sound (#6721)

[Keyboard] Fix default keymaps for OLKB boards to play Startup Sound (#6721)

* Fix Planck default keymap to play sounds on rev6

The dip_switch_update callback was overriding the default startup sound.  This should prevent that from happening, and still allow it to play sounds, or stop them, when appropriate.

* Fix Preonic default keymap to play sounds on Rev 3

The dip_switch_update callback was overriding the default startup sound.  This should prevent that from happening, and still allow it to play sounds, or stop them, when appropriate.