powerpc/64s: Fix Power9 DD2.1 logic in DT CPU features

I got the logic wrong in the DT CPU features code when I added the
Power9 DD2.1 feature. We should be setting the bit if we detect a
DD2.1, not clearing it if we detect a DD2.0.

This code isn't actually exercised at the moment so nothing is
actually broken.

Fixes: 3ffa9d9e ("powerpc/64s: Fix Power9 DD2.0 workarounds by adding DD2.1 feature")
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>