This patch updates the dequeueing logic to dequeue all requests at once.
Since we can have many requests in the queue, the interrupt coalescing
is kept so that the ring interrupt fires every EIP197_MAX_BATCH_SZ at
most.

To allow dequeueing all requests at once while still using reasonable
settings for the interrupt coalescing, the result handling function was
updated to setup the threshold interrupt when needed (i.e. when more
requests than EIP197_MAX_BATCH_SZ are in the queue). When using this
capability the ring is marked as busy so that the dequeue function
enqueue new requests without setting the threshold interrupt.
Suggested-by: Ofer Heifetz <oferh@marvell.com>
Signed-off-by: Antoine Tenart <antoine.tenart@free-electrons.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>