I'm pretty sure all callers rely on this command to return either 0 or a
negative errno. There's a quirky occasion where if we get a HID error (a
positive number) during hidpp20_special_keys_buttons_get_reporting, that
number eventually ends up as the 'num_controls' value in
hidpp20_special_key_mouse_get_controls().
Since the only time we care about HID++ errors is during
hidpp20_root_get_protocol_version, let's just always return -EPROTO on HID++
errors.
Fixes#558
Signed-off-by: Peter Hutterer <peter.hutterer@who-t.net>
3cd5a1f986