powerpc/msi: Use WARN_ON() in msi bitmap selftests

As demonstrated in the previous commit, the failure message from the msi
bitmap selftests is a bit subtle, it's easy to miss a failure in a busy
boot log.

So drop our check() macro and use WARN_ON() instead. This necessitates
inverting all the conditions as well.

Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>