r8169: replace custom flag with disable_work() et al

So far we use a custom flag to define when a task can be scheduled and
when not. Let's use the standard mechanism with disable_work() et al
instead.
Note that in rtl8169_close() we can remove the call to cancel_work()
because we now call disable_work_sync() in rtl8169_down() already.

Signed-off-by: Heiner Kallweit <hkallweit1@gmail.com>
Signed-off-by: Andrew Lunn <andrew@lunn.ch>