crypto: marvell/cesa - rearrange handling for hw finished hashes

Rearrange the last request handling for hardware finished hashes
by moving the generation of the fragment operation into this path.
This results in a simplified sequence to handle this case, and
allows us to move the software padded case further down into the
function.  Add comments describing these parts.

Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>