crypto: crypto4xx - fix stalls under heavy load

If the crypto4xx device is continuously loaded by dm-crypt
and ipsec work, it will start to work intermittent after a
few (between 20-30) seconds, hurting throughput and latency.

This patch contains various stability improvements in order
to fix this issue. So far, the hardware has survived more
than a day without suffering any stalls under the continuous
load.

Signed-off-by: Christian Lamparter <chunkeey@gmail.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>