Fix pressure reporting when tool goes out of proximity

Currently we are reporting a pressure of 1 << (6 + extra_z_bits) when no tools
are in the proximity. This commit makes us report -1 instead in that case,
as the usb wacom driver does.

Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Add support for scroll wheel on Graphire mouse

Signed-off-by: Hans de Goede <hdegoede@redhat.com>