dell-wmi: don't generate errors on empty messages

There's no point in generating kernel messages if we didn't receive a
parsable keyboard event - only do so if there appeared to be a scancode.

Signed-off-by: Matthew Garrett <mjg@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Len Brown <len.brown@intel.com>