crypto: atmel-sha - update request queue management to make it more generic

This patch is a transitional patch. It splits the atmel_sha_handle_queue()
function. Now atmel_sha_handle_queue() only manages the request queue and
calls a new .start() hook from the atmel_sha_ctx structure.
This hook allows to implement different kind of requests still handled by
a single queue.

Also when the req parameter of atmel_sha_handle_queue() refers to the very
same request as the one returned by crypto_dequeue_request(), the queue
management now gives a chance to this crypto request to be handled
synchronously, hence reducing latencies. The .start() hook returns 0 if
the crypto request was handled synchronously and -EINPROGRESS if the
crypto request still need to be handled asynchronously.

Besides, the new .is_async member of the atmel_sha_dev structure helps
tagging this asynchronous state. Indeed, the req->base.complete() callback
should not be called if the crypto request is handled synchronously.

Signed-off-by: Cyrille Pitchen <cyrille.pitchen@atmel.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>