crypto: powerpc - Fix initialisation of crc32c context

Turning on crypto self-tests on a POWER8 shows:

    alg: hash: Test 1 failed for crc32c-vpmsum
    00000000: ff ff ff ff

Comparing the code with the Intel CRC32c implementation on which
ours is based shows that we are doing an init with 0, not ~0
as CRC32c requires.

This probably wasn't caught because btrfs does its own weird
open-coded initialisation.

Initialise our internal context to ~0 on init.

This makes the self-tests pass, and btrfs continues to work.

Fixes: 6dd7a82c

 ("crypto: powerpc - Add POWER8 optimised crc32c")
Cc: Anton Blanchard <anton@samba.org>
Cc: stable@vger.kernel.org
Signed-off-by: Daniel Axtens <dja@axtens.net>
Acked-by: Anton Blanchard <anton@samba.org>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>