crypto: marvell/cesa - rearrange last request handling

Move the test for the last request out of mv_cesa_ahash_dma_last_req()
to its caller, and move the mv_cesa_dma_add_frag() down into this
function.

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