Input: xpad - remove mouse buttons and axes for dance pads

Dance pads don't have any axes/sticks, only buttons for directions. For
example buttons like left+right will get triggered at once, an axis
can't handle this anyway.  So this patch adds a module parameter named
"sticks_to_null" for unknown devices. A known dance pad makes use of
it by changing to a new mapping-option named DANCEPAD_MAP_CONFIG. Other
tested devices may follow by adding this mapping-option too.

Some buttons of xpad-devices are addressing mouse-buttons instead of
gamepad-buttons. This gets fixed too.

Signed-off-by: Christoph Fritz <chf.fritz@googlemail.com>
Signed-off-by: Dmitry Torokhov <dtor@mail.ru>