fallback: explicitly ignore external keyboards for the tablet mode switch

We already had a check to only pair trackpoints and internal keyboards
but for the ThinkPad Compact Bluetooth Keyboard with TrackPoint that isn't
sufficient - it's an external keyboard that contains a trackpoint. Explicitly
ignore external keyboard, we never want to shut those down in tablet mode
anyway.

Fixes #119

Signed-off-by: Peter Hutterer <peter.hutterer@who-t.net>