workqueue: Replace usage of init_name with dev_set_name()

The init_name property of the device struct is sort of a hack and should
only be used for statically allocated devices. Since the device is
dynamically allocated here it is safe to use the proper way to set a
devices name by calling dev_set_name().

Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Signed-off-by: Tejun Heo <tj@kernel.org>