selftests/powerpc: Remove repeated word in comments

Remove the repeated word "not" in comments.

Signed-off-by: Shaomin Deng <dengshaomin@cdjrlc.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20221029094643.5595-1-dengshaomin@cdjrlc.com