workqueue: fix dev_set_uevent_suppress() imbalance

Uevents are suppressed during attributes registration, but never
restored, so kobject_uevent() does nothing.

Signed-off-by: Maxime Bizon <mbizon@freebox.fr>
Signed-off-by: Tejun Heo <tj@kernel.org>
Cc: stable@vger.kernel.org
Fixes: 226223ab