touchpad: a touchpad with only one button is a clickpad

touchpad: a touchpad with only one button is a clickpad

There is only one touchpad with a physical left button but no right button and
that is the old Apple touchpad, discontinued in 2008. Not a huge number of
those left, I assume.

So let's change our assumptions because these days the vast majority of
touchpads are clickpads - any touchpad that only has a left button is treated
as clickpad, even where the kernel doesn't set the INPUT_PROP_BUTTONPAD.

We do need to check for BTN_LEFT as well though, because Wacom touchpads (i.e.
the touch part of non-integrated Wacom tablets) don't have a left button
either.

Signed-off-by: Peter Hutterer <peter.hutterer@who-t.net>