powerpc: Fix pgtable pmd cache init

Commit 9b081e10 ("powerpc: port 64 bits pgtable_cache to 32 bits")
mixed up PMD_INDEX_SIZE and PMD_CACHE_INDEX a couple of times. This
resulted in 64s/hash/4k configs to panic at boot with a false positive
error check.

Fix that and simplify error handling by moving the check to the caller.

Fixes: 9b081e10

 ("powerpc: port 64 bits pgtable_cache to 32 bits")
Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>