Invert the DJ Hero Turntable crossfader axis bind.

[shockdude]

Back when I submitted turntable emulation in #9965, I set the crossfader bind such that Crossfade Left was Y Up and Crossfade Right was Y Down, intending to imitate the binds of an Xbox 360 turntable.

Turns out I messed up. On Xbox 360 turntables, Crossfade Left is Y Down, Crossfade Right is Y Up.
This PR flips the crossfader bind to match the real turntable's inputs.

This will flip the crossfader for all emulated turntable users until they rebind, but I'm familiar with the DJ Hero community and I think it's worth a one-time change for accuracy.

Also fix turntable/left stick input overflow while I'm here.

[shockdude]

This change also makes setting up Wii turntables using the WiitarThing driver much easier, as WiitarThing emulates a 360 controller and already outputs the correct axes for the turntable crossfader (using WiitarThing with current RPCS3 results in an inverted crossfader ingame)

[shockdude]

Is there anything else I need to do for this PR?

[Megamouse]

I mean i could merge it, but wouldn't it make more sense to understand the underlying issue with mmjoy vs xinput first?

[shockdude]

I mean i could merge it, but wouldn't it make more sense to understand the underlying issue with mmjoy vs xinput first?

I found the underlying issue, see #13671. Simplified this PR based on #13671 getting merged.

The check for buf[6] == 127 is still required in the case of joysticks centered at (128,128), but at least there's no funny behavior (subtracting from 256 instead of 255) due to joysticks centered at (126,126).