crypto: streebog - fix unaligned memory accesses

Don't cast the data buffer directly to streebog_uint512, as this
violates alignment rules.

Fixes: fe18957e ("crypto: streebog - add Streebog hash function")
Cc: Vitaly Chikunov <vt@altlinux.org>
Signed-off-by: Eric Biggers <ebiggers@google.com>
Reviewed-by: Vitaly Chikunov <vt@altlinux.org>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>