Input: clarify gamepad API ABS values

It wasn't really clear from the gamepad-API description how ABS values are
mapped exactly. Clarify that negative is left/up and positive is
right/down. Unfortunately, this means I screwed up the Wii U ProController
ABI. Anyhow, this just means we continue to have 0 compatible gamepad
drivers in the kernel. User-space needs to fix them up, anyway, as all
other gamepads are also incompatible.

Signed-off-by: David Herrmann <dh.herrmann@gmail.com>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>