crypto: omap-aes - Populate number of SG elements

Crypto layer only passes nbytes but number of SG elements is needed for mapping
or unmapping SGs at one time using dma_map* API and also needed to pass in for
dmaengine prep function.

We call function added to scatterwalk for this purpose in omap_aes_handle_queue
to populate the values which are used later.

Signed-off-by: Joel Fernandes <joelf@ti.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>