Rearrange the last request handling for hashes which require software
padding.

We prepare the padding to be appended, and then append as much of the
padding to any existing data that's already queued up, adding an
operation block and launching the operation.

Any remainder is then appended as a separate operation.

This ensures that the hardware only ever sees multiples of the hash
block size to be operated on for software padded hashes, thus ensuring
that the engine always indicates that it has finished the calculation.
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>