HID: wiimote: fix DRM debug-attr to correctly parse input

We need to correctly zero-terminate the input to parse it. Otherwise, we
always end up interpreting it as numbers.
Furthermore, we actually want hexadecimal numbers instead of decimal. As
it is a debugfs interface, we can change the API at any time.

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