HID: pidff: Define values used in pidff_find_special_fields

Makes it clear where did these values came from

Signed-off-by: Tomasz Pakuła <tomasz.pakula.oficjalny@gmail.com>
Reviewed-by: Michał Kopeć <michal@nozomi.space>
Reviewed-by: Paul Dino Jones <paul@spacefreak18.xyz>
Tested-by: Paul Dino Jones <paul@spacefreak18.xyz>
Tested-by: Cristóferson Bueno <cbueno81@gmail.com>
Tested-by: Pablo Cisneros <patchkez@protonmail.com>
Signed-off-by: Jiri Kosina <jkosina@suse.com>