r8169: improve rtl_schedule_task

The current implementation makes the implicit assumption that if a bit
is set, then the work is scheduled already. Remove the need for this
implicit assumption and call schedule_work() always. It will check
internally whether the work is scheduled already.

Signed-off-by: Heiner Kallweit <hkallweit1@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>