net: octeontx-83xx: Replace flush_scheduled_work() with flush_work()

After the commit c4f135d6 ("workqueue: Wrap flush_workqueue()
using a macro"), a compile warning would be emitted for invocation of
flush_scheduled_work() in order to avoid the usage of this function
anymore. It really makes no sense to flush system_wq in octeontx-83xx,
just flushing the sso->mbox_work should be enough.

Signed-off-by: Kevin Hao <kexin.hao@windriver.com>
Signed-off-by: Bruce Ashfield <bruce.ashfield@gmail.com>