tty, workqueue: remove keventd_up() usage

Now that workqueue can handle work item queueing from very early
during boot, there is no need to delay schedule_work() while
!keventd_up().  Remove it.

Signed-off-by: Tejun Heo <tj@kernel.org>
Cc: Jiri Slaby <jslaby@suse.cz>
Cc: Alan Cox <alan@linux.intel.com>