If we know that the tablet mode switch is bogus anyway, filter the event and
don't pass it to the caller. They won't know whether it's bogus so the only
result we get here is buggy behaviour.

This is the simplest solution here, it filters the mode switch at the lowest
level and thus the caller won't know that the tablet even has a mode switch at
all. Where the device doesn't have any other switches it'll also lose the
switch capability.

This may cause issues in some niche cases where the event node only has
that one bit and we now disabled it leaving us with a zero-event bit device.
Shouldn't matter to callers, but let's see.

Fixes #491



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