HID: nintendo: fix gencon button events map

This fixes the button event map to match the 3-button recommendation
as well as the redundant 'z' in the button map events for the Sega
MD/Gen 6 Button.

Signed-off-by: Ryan McClelland <rymcclel@gmail.com>
Reviewed-by: Daniel J. Ogorchock <djogorchock@gmail.com>
Signed-off-by: Jiri Kosina <jkosina@suse.com>