From rockbox manual:
3.1.3. Starting the original firmware
Rockbox has a dual-boot feature. To boot into the original firmware, shut down the
device as described above.Turn on the Hold switch immediately after turning the
player on. The Apple logo will display for a few seconds as Rockbox loads the original
firmware.
You can also load the original firmware by shutting down the device, then clicking the
Hold switch on and connecting the iPod to your computer.
Regardless of which method you use to boot to the original firmware, you can return
to Rockbox by pressing and holding Menu and Select simultaneously until the player
hard resets.