async: simplify lowest_in_progress()

The code in lowest_in_progress() are duplicated in two branches,
simplify them.

tj: Minor indentation adjustment.

Signed-off-by: Lai Jiangshan <laijs@cn.fujitsu.com>
Signed-off-by: Tejun Heo <tj@kernel.org>
Cc: Arjan van de Ven <arjan@linux.intel.com>