Input: xpad - do not report nonexistent buttons for xbox360

The buttons BTN_C and BTN_Z are only used in the original xbox
controller, not in xbox360 controller. Therefore only add them to
keybit when the controller is a non-360 one.

Signed-off-by: Anssi Hannula <anssi.hannula@gmail.com>
Signed-off-by: Dmitry Torokhov <dtor@mail.ru>