PenMount 6000 USB resistive touchscreen controller reports it has three
buttons, while in reality it doesn't have any and doesn't support active
styli, and only generates touch events.

In penmount_input_mapping(), map only the first button (with code 0),
ignore the rest.
Signed-off-by: Andrew Shadura <andrew.shadura@collabora.co.uk>
Tested-by: Christian Gmeiner <c.gmeiner@bachmann.info
Signed-off-by: Jiri Kosina <jkosina@suse.cz>