Input: iforce - reorganize joystick configuration lists

* btn_avb_pegasus and btn_avb_tw are the same. Unify them
  into btn_joystick_avb.

* btn_wheel is an exact copy of btn_joystick, so remove it.
  Rename btn_avb_wheel into btn_wheel since it is the only
  sane configuration for a wheel.

* Assign the (new) btn_wheel configuration to the "AVB Top
  Shot Force Feedback Racing Wheel", because the previous
  configuration was meant for a joystick.

Signed-off-by: Tim Schumacher <timschumi@gmx.de>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>