dell-wmi - fix condition to abort driver loading

From: Dmitry Torokhov <dmitry.torokhov@gmail.com>

The commit 1fdd407f incorrectly made driver
abort loading when known GUID is present when it should have done exactly
the opposite.

Signed-off-by: Dmitry Torokhov <dtor@mail.ru>
Signed-off-by: Len Brown <len.brown@intel.com>