HID: wacom: Replace magic masks and comparisons with switch cases

Reasoning through the conditions under which a particular block of code
in 'wacom_intuos_general' will be reached is not at all easy due to the
sheer number of magic masks and comparisons. Remove these and replace
them with a switch statement over the various 'types' of packets that
will be encountered.

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