Input: wacom - use unaligned access where necessary

A few cases of incorrectly using 'le16_to_cpup' instead of
'get_unaligned_le16' have been noticed and fixed.

Signed-off-by: Jason Gerecke <killertofu@gmail.com>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>