crypto: gcm - use correct endianness type in gcm_hash_len()

In gcm_hash_len(), use be128 rather than u128.  This fixes the following
sparse warnings:

    crypto/gcm.c:252:19: warning: incorrect type in assignment (different base types)
    crypto/gcm.c:252:19:    expected unsigned long long [usertype] a
    crypto/gcm.c:252:19:    got restricted __be64 [usertype]
    crypto/gcm.c:253:19: warning: incorrect type in assignment (different base types)
    crypto/gcm.c:253:19:    expected unsigned long long [usertype] b
    crypto/gcm.c:253:19:    got restricted __be64 [usertype]

No actual change in behavior.

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