r8169: use RTNL to protect critical sections

Most relevant ops (open, close, ethtool ops) are protected with RTNL
lock by net core. Make sure that such ops can't be interrupted by
e.g. (runtime-)suspending by taking the RTNL lock in suspend ops
and the PCI error handler.

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