crypto: marvell/cesa - use presence of scatterlist to determine data load

Use the presence of the scatterlist to determine whether we should load
any new user data to the engine.  The following shall always be true at
this point:

	iter.base.op_len == 0 === iter.src.sg

In doing so, we can:

1. eliminate the test for iter.base.op_len inside the loop, which
   makes the loop operation more obvious and understandable.

2. move the operation generation for the cache-only case.

This prepares the code for the next step in its transformation, and also
uncovers a bug that will be fixed in the next patch.

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