Input: xpad - change buttons the D-Pad gets mapped as to BTN_DPAD_*

Input: xpad - change buttons the D-Pad gets mapped as to BTN_DPAD_*

Since dance pads can have both up/down or left/right pressed at the same time,
by design, they are not suitable for mapping the buttons to axes. Historically,
this driver mapped the D-pad to BTN_TRIGGER_HAPPY1-4 in these cases, and before
that as mouse buttons. However, BTN_DPAD_* exists for this and makes far more
sense than the arbitrary mapping it was before.

Signed-off-by: Vicki Pfau <vi@endrift.com>
Link: https://lore.kernel.org/r/20250702034740.124817-1-vi@endrift.com
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>