workqueue: cosmetic update in try_to_grab_pending()

With the recent is-work-queued-here test simplification, the nested
if() in try_to_grab_pending() can be collapsed.  Collapse it.

This patch is purely cosmetic.

Signed-off-by: Tejun Heo <tj@kernel.org>
Cc: Lai Jiangshan <laijs@cn.fujitsu.com>