crypto: stm32 - check request size and scatterlist size when using DMA.

When we are sending the data to HASH with the DMA, we send all the data
provided in the scatterlists of the request.
But in some cases (ex : tcrypt performances tests), we should only send
req->nbytes
When iterating through the scatterlist we verify if it is the last
scatterlist or if the number of bytes sent plus the data of the current
scatterlist is superior of the total number of bytes to hash.

Signed-off-by: Thomas Bourgoin <thomas.bourgoin@foss.st.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>