ioc4: use static work_struct for ioc4_load_modules()

There is no reason to dynamically allocate work_struct for
ioc4_load_modules().  It makes the code more complex and makes it
impossible to flush the work directly.  Use static work
ioc4_load_modules_work instead and flush it directly on exit.

This removes the use of flush_scheduled_work() which is being
deprecated.

Signed-off-by: Tejun Heo <tj@kernel.org>
Cc: Brent Casavant <bcasavan@sgi.com>