xwiishow: print N/A for invalid IR values
xwiishow: print N/A for invalid IR values

Use new xwii_event_ir_is_valid() to check whether IR values are valid and
mark them as N/A if they are not.

Signed-off-by: David Herrmann <fb53b2ddb8d141e0cb39d7c67c2f81b6bc2eb0f7@gmail.com>

barracks510/xwiimote,barracks510/xwiimote,dvdhrm/xwiimote,AdamLuchjenbroers/xwiimote,dvdhrm/xwiimote,barracks510/xwiimote,barracks510/xwiimote,AdamLuchjenbroers/xwiimote