dell-wmi: Improve unknown hotkey handling

If DMI lists a hotkey that we don't recognize, log and ignore it
instead of trying to map it to keycode 0.  I haven't seen this happen,
but it will help maintain the key map in the future and it will help
avoid sending bogus events.

This also improves the message that we log when we get an unknown key
event.

Signed-off-by: Andy Lutomirski <luto@kernel.org>
Reviewed-by: Pali Rohár <pali.rohar@gmail.com>
[dvhart: remove BUILD_BUG_ON per mutual agreement on list]
Signed-off-by: Darren Hart <dvhart@linux.intel.com>