powerpc/msi: Fix the msi bitmap alignment tests

When we added the alignment tests recently we failed to check they were
actually passing - oops.

They weren't passing, because the bitmap was full. We should also be a
bit more careful when checking the return code, a negative error return
could by divisible by our alignment value.

Fixes: b0345bbc

 ("powerpc/msi: Improve IRQ bitmap allocator")
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>