crypto: sahara - replace tasklets with kthread

In preparation for SHA support, replace the tasklets with a kthread that
manages one crypto_queue for the core.

As the Sahara can only process one AES or SHA request at a time, we make
sure that the queue serializes all requests from userspace. Instead of a
watchdog timer we now use a completion mechanism in the queue manager
thread.
This makes the control flow more obvious and guarantees, that only one
request is dequeued until the completion is completed.

Signed-off-by: Steffen Trumtrar <s.trumtrar@pengutronix.de>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>