crypto: gf128mul - constify 4k and 64k multiplication tables

Constify the multiplication tables passed to the 4k and 64k
multiplication functions, as they are not modified by these functions.

Cc: Alex Cope <alexcope@google.com>
Signed-off-by: Eric Biggers <ebiggers@google.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>