Fix SipHash init order.

Setting the SipHash hash size and setting its key is done with two
independent functions...  and yet, the internals depend on both.

Unfortunately, the function to change the size wasn't adapted for the
possibility that the key was set first, with a different hash size.

This changes the hash setting function to fix the internal values
(which is easy, fortunately) according to the hash size.

evpmac.txt value for digestsize:8 is also corrected.

Reviewed-by: Paul Dale <paul.dale@oracle.com>
(Merged from https://github.com/openssl/openssl/pull/7613)
