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>