r8169: Fix runtime power management

I noticed that one of the post-2.6.36 patches broke runtime PM of the
r8169 on my MSI Wind test machine in such a way that the link was not
brought up after reconnecting the network cable.

In the process of debugging the issue I realized that we only should
invoke the runtime PM functions in rtl8169_check_link_status() when
link change is reported and if we do so, the problem goes away.
Moreover, this allows rtl8169_runtime_idle() to be simplified quite
a bit.

Signed-off-by: Rafael J. Wysocki <rjw@sisk.pl>
Acked-by: Francois Romieu <romieu@fr.zoreil.com>
Signed-off-by: David S. Miller <davem@davemloft.net>