HID: wiimote: discard invalid EXT data reports

If an extension device isn't initialized properly, or during hardware
initialization, a device might send extension data which is all 0xff.
This is ambigious because this is also a valid normal data report. But
it is impossible, under normal conditions, to trigger valid reports with
all 0xff. Hence, we can safely ignore them.

Signed-off-by: David Herrmann <dh.herrmann@gmail.com>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>