Input: appletouch - simplify touchpad type detection

This patch simplifies type detection and removes unnecessary code.

Signed-off-by: Sven Anders <anders@anduras.de>
[jberg: don't typedef, checkpatch clean, remove useless comments, ...]
Signed-off-by: Johannes Berg <johannes@sipsolutions.net>
Signed-off-by: Dmitry Torokhov <dtor@mail.ru>