touchpad: refactor our clickpad assignments

Previously, we used INPUT_PROP_BUTTONPAD as the only reliable source for whether a device was a clickpad or not. This is no longer the case, several devices in recent history have had that property either wrongly set or wrongly unset, see b925a087 and 69959c8a. We logged errors in that case, but otherwise continued as-is.

Let's change that and use the presence of buttons are the final arbiter. We still log an error but otherwise the presence of a right button will decide whether we assume a device is a clickpad.

Note that this only applies to devices without a quirk set, ideally all devices that produce an error will eventually set the quirk so the error is no longer there (or to change the logic where the guess is wrong).

cc @bentiss in case he remembers whether we have any additional things to worry about here

Fixes #595 (closed)