HID: wacom: Clean up value reading

Make the logic for reading X, Y, distance, and pressure a bit more
clear. An additional bit was stuffed into the packet format many
models back, and /most/ devices in use will use it. If we happen
to be dealing with a particularly old tablet, just shift it off
the end to pretend we never read it.

Signed-off-by: Jason Gerecke <jason.gerecke@wacom.com>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>