crypto: sun4i-ss - simplify the pad length calculation

When sending the last block of data to the engine, it should be padded
so that the total length of the request can be given to the engine as
the last 2 words of the last 64 bytes block.

Simplify the calculation of this pad offset.

Signed-off-by: Antoine Tenart <antoine.tenart@free-electrons.com>
Tested-by: Corentin Labbe <clabbe.montjoie@gmail.com>
Acked-by: Corentin Labbe <clabbe.montjoie@gmail.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>